General fixes

I've bumped the gradle plugin versions, build tool version. 
Also I had to add  okhttp 1.6.0 and okhttp-urlconnection 1.6.0 because Picasso was raising an Exception. ( Detail [here](http://stackoverflow.com/questions/24125856/picasso-detected-an-unsupported-okhttp-on-the-classpath) )

At least, I am able to build it and working for me now. Thanks a lot for open sourcing it.