Implement feature extraction

[gsergiu]

The MIR features extraction must be implemented in java and place in the mir-features project.
Each feature extractor must implement serialization to JSON and a unit test has to be implemented.
A sample file was added in the test resources.

[gsergiu]

I recommend using GSON library for json serialization:
http://mvnrepository.com/artifact/com.google.code.gson/gson