[runtime] Accumulators are reported through the RuntimeEnvironment, not directly sent as an actor message

This commit adds a figure giving an overview of Flink's stack. Users can click
on components to go to the respective component docs.

This closes #585.

This closes #563.

[quickstarts] Set different default groupid for quickstarts to avoid exclusion by the assembly plugin

This closes #544

This closes #547

[FLINK-1847] [Scala API] Changes the return type of Scala API's collect method to Seq[T] and adds parentheses to collect and count methods.

 - Updates existing test cases using Scala API's collect/count method.

This closes #583

[FLINK-1845] [runtime] Fixup ReusingKeyGroupedIterator value iterators in NonReusingSortMergeCoGroupIterator.

This closes #559

This closes #555.

Closes #577

Instead of a checking against a concrete output it only determines well-formedness.

The job is now commutative and associative and thus testable in parallel.

This closes #578

This closes #575

[licences] Updated LICENSE and NOTICE file of the binary distribution to contain dependencies of 'flink-dist'.

This means that the LICENSE and NOTICE files reflect all dependencies that we include
in the binary distribution, but not those of artifacts pulled only through maven.

Running package locally resulted in the following error:

[ERROR] Failed to execute goal ... on project flink-streaming-connectors: Compilation failure
[ERROR] .../KafkaTopicUtils.java:[57,25] cannot access org.apache.zookeeper.Watcher
[ERROR] class file for org.apache.zookeeper.Watcher not found

Mysteriously, this does not happen on Travis.

Checkpointing currently does not support this special case

This closes #519

This closes #565

Added Windows test cases to Path tests

This closes #491

Also rename jobmanager.TaskManagerRegistrationTest to avoid name conflicts.