Add support of logging in object level

[Jintin]

Add support of object level logging in order to increase readability for case like binary response we can't log in OkHttp layer.

High level changes:
Add one interface ObjectLogger with two implementation EmptyLogger and GsonLogger which print all attribute via gson.
Add new function setObjectLogger in Retrofit.Builder in order to set ObjectLogger from outside.
Add logging in SimpleService to test/verify easily.

Usage sample here:

Retrofit retrofit = new Retrofit.Builder()
    .baseUrl("https://api.example.com")
    .setObjectLogger(GsonLogger.create())
    .build();

Please let me know if there's any comment.
Thanks.

[Jintin]

btw I guess Android related build failed is normal?