Requirejs enhancements

The development group I work with has been using your js plugin for gradle to run the require optimizer.

Recently, I ran into a problem optimizing a file that had already been optimized.  It turns out that the require engineers made a change recently to address this, but the version of r.js embedded in the plugin (2.1.4) was before that change.

I don't know exactly what version has the fix, but the latest r.js version is 2.1.8.

In this change set, I upgraded the embedded r.js to be the latest file.  I also added a new configuration option for the requireJs task (requirejs.impl) that allows users to specify an alternative version of r.js in their build.gradle file.  This way, if people need to use an older or newer version, they can specify it themselves without having to touch the plugin like I did.

I also added unit tests and updated the Readme, as suggested in your contribution guide.

Hope you like it, let me know if you have any questions.