Add support for publishing to Bintray

This pull request configures the build of this project such that you can publish your releases to bintray. Bintray artifact hosting is free to open source projects. You can also configure this project to sync to jcenter as well for wider availability.

Sign up for an account on bintray (https://bintray.com/) and setup a api key for automating uploads. Best practice for managing this key (and your username) is to store them in your gradle.properties file in the .gradle folder of of your user home directory. This will keep it so that you do not have to include them in the repository on github.

Once you have uploaded your first version of this artifact to you bintray account, you can configure this artifact to sync to JCenter. This is done by clicking the link from the artifacts page on bintray. Requests for syncing to JCenter are almost always granted without additional requests and are ususally completed within a few hours.

To support this new function, I changed this build from using the maven plugin to using the maven-publish plugin. This effectively removes the 'install' task you are used to using, but the same behavior is available in the 'publishToMavenLocal' task.

Also added the release plugin so that code releases can be handled with a single command. More information on this plugin can be found here:

https://github.com/townsfolk/gradle-release