Added support for px-based letter-spacing and word-spacing

Letter and word spacing using pixels causes FitText to behave badly. Setting those spacing values to scale along with the font-size is relatively trivial, so I added that in.  

Contributed code appears to work consistently in the sample of desktop browsers I tested against:

Ubuntu (12.04):
- Chrome v25.0.1364.160
- FF 21.0

XP:
- Chrome v27.0.1453.110 m
- Chrome v28.0.1500
- FF 20.0.1
- FF 22
- IE 8.0.6.001.18702

Testing on mobile devices worked, but was dependent on native typefaces and required some manual adjustment of the compressor parameter.  May be best to adjust example.html to use a Google font that renders somewhat more consistently?

EDIT1: Formatting.
EDIT2: Clarified this is for pixel-based units.