OkHttpClient & Workaround for certificate pinning on android 2.3

- Updated the retrofit client to use the 2.0 OkHttpClient (merged the development branch into my master branch)
- Fixed an issue with certificate pinning on android 2.3 

Please note, you need to change the strings.xml and update it with your oauth token to make a correct request. Although a 401 response is also good, since it means SSL was working.

```
<string name="oauth_token">YOUR_TOKEN_HERE</string>
```