Added support to override the r.js (require optimizer) implementation via a new 'impl' option
Move current copy of r.js into src/test tree to support new unit test for requireJs 'impl' option
Upgraded r.js from 2.1.4 to 2.1.8
Changed 'impl' to 'requirejs.impl' to be more consistent with buildprofile
Merge branch 'requirejs-enhancements' of https://github.com/MartinSnyder/gradle-js-plugin into requirejs-enhancements