Plugable extension framework for support C++ map/reduce/collector/combiner.

[clockfly]

We should be able to support any customer mapper and customer reducer.

We should design and develop a well defined C++ extension frameworks. The user can develop C++ .so libs based on our documents, and submit the .so along with the job jar.

[clockfly]

Seems it is already supported.

But we need to docuemnt the usage, and develop an example for this.