Many changes to the drawText() method, along with a few other changes

- Rewrote the text-wrapping algorithm to be more bulletproof (hopefully)
- Added new font properties for the drawText() method: 'fontStyle',
'fontSize', and 'fontFamily'. These properties can be used (and are
preferred) instead of the old 'font' property
- Layer indices can now be negative (-1 signifying the last layer in
the layers array) for all jCanvas layer methods
- Fixed a bug which caused the drawLayers() method to throw an error if
called on a non-canvas element
Fixed a bug in which an image drawn from a DOM element could not be resized
Merge branch 'master' of https://github.com/caleb531/jcanvas