Notify when less is done processing

Fixes issue #283 "How to tell when less is done processing ?"

**Problem**
When using less.js in the browser, it is sometimes necessary to wait for less.js to finish processing and applying new css values before running additional code or tests.

**Use cases**
* Executing code that is depending on less.js having finished processing.
* Delaying E2E tests for an in browser theme customizer until less.js has completed its refresh().
* Avoiding FOUC by hiding elements and displaying them after less.js has completed.

**Solutions** (thus far proposed)
* Fire an event after procesing.
* Call a callback function after LESS is done processing.
* Have less.refresh() return a promise.