Move build to Gradle

Here's a stab at moving the build from ant to gradle.

BUILDING.txt contains some basic instructions.

I've added some stuff to get us on the way to getting this in to Maven Central, but one of you guys will need to actually do the signing and publishing.

[This guide](http://jedicoder.blogspot.co.uk/2011/11/automated-gradle-project-deployment-to.html) should be a good start. I've done this before on OSX though, not windows.