Support JsonAdapter Annotation

[yasirmhd]

Support JsonAdapter annotation which is also available in Gson. The complete documentation about JsonAdapter is available here : https://github.com/google/gson/blob/master/gson/src/main/java/com/google/gson/annotations/JsonAdapter.java

There are some adapters which are very custom and need to be hand-written. Given this support the adapter for these classes will also be recognised by STAG while generating the code.

This will also solve the scenario mentioned in issue #25

[yasirmhd]

Will pick this up next week.

[anirudhramanan]

#72

[anthonycr]

Closing as it has been added in #72