[streaming] Extended test cases for grouped window invokable to cover usage of central eviction policies

[streaming] Renamed test cases for window and grouped window invokable to be consistent with the invokable names

[streaming] Changed time based windowing policies to make lower boundary excluded and upper boundary included in the windows and adjusted policy test cases respectively.

[streaming] Make all predefined policies cloneable and introduce an activating wrapper for cloneable eviction policies

[streaming] Introduced grouped windowing invokable, added required cloneable policy interfaces, and adjusted existing windowing invokable to make it usable together with the new grouped windowing invokable.

[streaming] Fixed windowing examples by removing manual setting of source parallelism

[streaming] Extended streaming guide with a section about the new policy based windowing.

[streaming] Integrated the policy based windowing into DataStream and introduced WindowedDataStream to handle windowing helper.

[streaming] Added cosine distance and Euclidean distance as examples for possible delta function implementations. Test cases are included as well.

[streaming] Added policy helper for delta policies to provide a simpler API for such policies.

[streaming] Created ActiveEvictionPolicyWrapper. This eviction policy wrappes around a non-active policy and makes it active by forwarding notification on fake elements to the regular notification method.

[streaming] Added abstract class ExtractionAwareDeltaFunction to enable easy extraction of data in pre-defined delta functions.

[streaming] Created trigger and eviction policies which calculate a delta between two data points using a given delta function. The policies evict/trigger in case the delta is higher than a specified threshold. (incl. test case and the interface for delta functions)

[streaming] Created tumbling eviction policy. This policy evicts always all tuples after a trigger occurred. It can be used to prevent doubled computation between trigger and eviction policies whenever tumbling windows are used. (includes test case)

[streaming] Added extractor implementations for various extractions and conversions. (incl. test cases)

[streaming] Created active trigger and eviction policies based on time. Included respective policy helper and test cases.

[streaming] Introduced extractor interface to enable flexible handling of arbitrary input data types.

[streaming] Created trigger and eviction policies based on element counters. Included respective policy helper and test cases.

[streaming] Added interface for policy helpers. Such helpers will allow to use policy based windowing through a simpler API.

[streaming] Introduced a new invokable to allow flexible windowing based on trigger and eviction policies. Additionally created all required policy interfaces.

- Move some background on the most important configuration values into the configuration guide
- Remove the term "UDF" because it is often being confused with UDFs in SQL
- Fixe some typos

This closes #244.

This closes #241

The code originates from https://github.com/apache/incubator-flink/pull/61.
The pull request also contained code to add POJO support to the distinct() operator. This has already been implemented in earlier work

This closes #233

This closes #240.

This closes #242

Add optional test for external sorting of case classes.
Fix various warnings.

This closes #238