Use retrofit rewrite network related codes

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I realize retrofit is better than our implementation. Meanwhile, we may need to use gson.

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Why

Because current codes have poor error handle, poor multiple threads

What packages to use

1. Retrofit: http://square.github.io/retrofit/
2. RxJava: https://github.com/ReactiveX/RxJava
3. Gson: compile 'com.squareup.retrofit:converter-gson:2.0.0-beta1'
4. OkHttp

Reference:

Retrofit 2.0: The biggest update yet on the best HTTP Client Library for Android

Retrofit and RxJava, Android multi-threaded REST requests

[xcv58]

Almost finish.