Added HTTP PATCH support to the library

To implement an API we use I've added HTTP PATCH support to the library. I think this is a welcome addition.

To support HTTP PATCH including the Apache httpclient.jar in your project is required, I stated this in the Readme.md so everybody knows what to do. Note that everything keeps working as it does now when you don't include the extra jar. 

So I think it's a nice addition for people that need it, those who don't need it don't need to include extra stuff.

Hope you will pull this in. If there is anything you need to have changed before you can pull it in, let me know. I'm happy to tweak and change some things around until you're satisfied.