Add a request factory based on okhttpclient

Add a request factory based on okhttpclient if okhttpclient is available on the classpath. https://github.com/SpringSource/spring-android/issues/10

This pull requests add very little code and provides an [okhttp](http://square.github.io/okhttp/) based implementation of spring android. This http engine has several advantages : 
* SPDY support allows all requests to the same host to share a socket.
* Connection pooling reduces request latency (if SPDY isn’t available).
* Transparent GZIP shrinks download sizes.
* Response caching avoids the network completely for repeat requests.

The pull requests is simple and is tested (thx to your tests). 

I also updated the android maven plugin version to get things working with latest android SDK.