Upgraded less.js to version 1.7.2 and added support for nashorn (Java 8)

 * Pulled changes to `less-rhino.js` from upstream
 * Replaced `lessc-rhino.js` from upstream with our own version that is much simpler.
 * Redesigned compiler/engine functions to support both rhino and nashorn (JDK 7/8).
 * Added some tests for compile/engine behaviour.
 * Improved error handling and reporting, addresses #2.