Gson as maven dependency

@MyMalcom With this change the app developer can add gson dependency together with malcom.

This is an example from GymForLess. These two dependencies use gson 2.2.4:

```
dependencies {
    compile 'com.squareup.retrofit:retrofit:1.3.0'
    compile 'com.malcom.library.android:malcom-android-library:2.0.10'
    ...
}
```