Fix for issue #13: Use Node if available

Hi!

This pull request enables the plugin to use NodeJS as a javascript engine for running the optimize script. The changes in a nutshell:

*  added a new maven module (nodejs-maven) that creates several .jar files each containing a node binary for one platform. These .jar files can be used in maven project as a dependency using the right platform classifier.
* added a new maven module (nodejs-maven-util) for extracting the node binary from the .jar file and executing a script.
* added a new configuration entry for the plugin so users can select either 'rhino' or 'nodejs' as a javascript engine.
* moved the plugin source to a subdirectory and created a module pom in the root to build the new nodejs modules easily.

So far I've only tested this successfully on macos-x86-64 so some additional testing on other platforms is necessary. Also I'm not sure if bundling nodejs this way does not violate the license of either nodeJS or requirejs-maven-plugin.