Image retrieval example

This pull request serves for two purposes.

First, Has/Get Blob/Layer methods are added to simplify feature extraction.

Second, the very natural next step is to apply the extracted features in practical applications, e.g. image retrieval. The demo can also be deemed as a baseline method.

Related issues:
#20: Extract the middle features
#112: pythonic export of features and params for wrapper
#139: About dump_network.cpp