Created by: gangliao

@wangkuiyi @reyoung @hedaoyuan Requirement from #255 (closed). This PR is completely implemented in CMake to support code coverage which mainly include three components.

1. filter source code for code coverage, such as, *.cc, *.cpp, etc.
2. make test and generate gcov data
3. convert *.gcov to json and upload it to coveralls. https://coveralls.io/github/baidu/Paddle

Currently, Paddle achieves 63% coverage, way to go !!!

If you want to do code coverage in your local machine, you can simply issue the following commands:

cmake -DON_COVERALLS=ON -DCOVERALLS_UPLOAD=OFF ..
make
make coveralls