Generating python protobuf for paddle.v2.framework

Python should be able to manipulate Protobuf message because:

1. Python's `create_op_creation_methods` take the `OpProto` array to
   generate all `op_creation_methods` in RunTime.

2. All `op_creation_methods` will create an `OpDesc` and pass it to
   Paddle C++ method `CreateOp` and return the Op handle.

Here is the list of what is added in this commit:

* Add `protobuf_generate_python` if it is not defined.
  * Before cmake 3.4, `protobuf_generate_python` is not defined. Just
    copy the implementation of that function in `protobuf.cmake`
* Add `py_proto_compile` function in `cmake/generic.cmake`.
  * It follows bazel's API interface.
    * https://github.com/pubref/rules_protobuf#rules
* Add an empty package named `paddle.v2.framework`, all python code of
  `paddle::framework` will be in that package.
* Generate protobuf's python module `__init__.py` by `touch` while
  compiling.
* Change setup.py.in, make `paddle.v2.framework.proto` uses the
  generated protobuf pythons.

Fix CI error on test_LayerGrad.LSTM

* We should not EXPECT_EQ between a float value and a int value.
  Use ASSERT_NEAR instead.

fix bugs for CrossChannelNormLayer

fix cmake errors

Correct GLOG CHECK in Paddle

stride pooling for max and average layer

Use CHECK instead of PCHECK, because PCHECK is used for errno.

Remove buggy BarrierStat

fix compile errors in tensor. 
`PlaceholderImpl::Deleter` maybe used in other modules. We can consider moving it to be `paddle::memory::Deleter`.

fix cmake error

Add DetectionOutputLayer and MultiBoxLossLayer.

The implementation of BarrierStat is buggy, and it is not necessary
for Paddle to diagnose which node in cluster is slow.

* add op_desc.proto

In Operator design, we need a proto message to describe an Operator.
Third-party language such as python can build this proto message and use
AddOp(const OpDesc& op_desc) of Paddle core to construct an Op in the
Network.

fix wrong including header-file in files in paddle/platform/dynload dir

port dyload to platform

FIX: Add eigen and any as INTERFACE deps

* init etcd cclient

* add etcd

* add etcd.go

* fix compile problem

* move code to etcd.go

* add etcd_lister.go for pserver client

* add etcd_client_test.go

* merge etcd_client_test and client_test

* refine client_test.go

* refine code

* format code

* add TODO and use interface instead of struct

* fix typo of initDesiredPservers

* optimize dir structure of go/pserver/client

* add a flag to config index for pserver

* follow comment

* fix path

* optimize code

* remove err in pserver NewEtcd

* restore comment about /ps_desired

Add target_link_libraries for cc_library

It will fix #2728.

Maybe it is silly to `target_link_libraries` for static library,
because a static library do not need to link other libraries. But
it will tell cmake how to propagate dependencies.

The solution comes from
[here](http://floooh.github.io/2016/01/12/cmake-dependency-juggling.html).

* Also change op_proto_test DEPS for testing this fix works.