Add bintray release script

I've added a gradle script that sets some properties that are necessary to publish to bintray and then it uses external scripts that make all the magic.

This is the first one: https://gist.github.com/hal9002/0772dffbd8a6a81bc176

This first script uses this other script https://gist.github.com/hal9002/691bbf3340dde3a4591d to generate all the files needed to publish to a maven repo (thanks to @blundell ) and it also sets some basic configuration for bintray and applies the bintray gradle plugin.

The first gist can be modified to allow more customisation if necessary

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:x: do not merge yet, I need to create an API key for the novoda account in bintray

But can be reviewed.