Spike moving to Retrofit

:rotating_light: Do not merge :rotating_light: 

This is a spike of what it would look like to move the SDK to Retrofit. 3597d9e is the only commit that contains the Retrofit related changes.

Until Retrofit 2.0 is released the signing is more complex than it needs to be because the `RequestInterceptor` does not have access to the URL. The signing should hopefully become simpler with 2.0.

By moving to Retrofit the API definition becomes much easer and adding a new route is as simple as adding

```java
@GET("/users")
public User getUser() throws RetrofitError;

@GET("/users")
public void getUser(Callback<User> callback);
```

to the `PxApi` interface and adding the associated models. Retrofit also provides synchronous and asynchronous support for free without needing to write `AsyncTask`s or services and deserialization to Java objects allowing the use of strongly typed, pre-defined objects, instead of working with `JSONObjects` and `JSONArray`s. 

This would also fix #4.