PVector update for angleMode and rotate

This pull requests includes all of the code for PVector to deal with angles in degrees when necessary.  It also renames rotate2D() to rotate().   It should close: https://github.com/lmccart/p5.js/issues/174.  I'll leave it open for now just in case.

The unit tests should be updated to test for angleMode in degrees as well.  I noticed that the [PVector unit test](https://github.com/lmccart/p5.js/blob/master/test/unit/math/pvector.js) is using a different syntax (BDD?) than [the other tests](https://github.com/lmccart/p5.js/blob/master/test/unit/math/random.js) (TDD?).  I assume I should port these over?  @evhan55