Clean up: strip trailing whitespaces.
Update build-dependency "maven-compiler-plugin" from v2.3.2 to v3.1
Use annotations instead of doclet tags for Mojo specification.
Use Collections.<String> emptyList() instead of Collections.EMPTY_LIST.
Add a bunch of configuration options to the mojo.

It is now possible to configure some parameters using the
"<configuration>" section of the javascript-compiler-plugin:

* languageIn  : The JavaScript language version accepted.
* languageOut : The JavaScript language version that should be produced.
* closurePass : Processes goog.provide() and goog.require() calls

.. more options might be added in the future...