Added support for letter-spacing and word-spacing

Letter and word spacing 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:

Webkit
  Ubuntu (12.04)
    Chrome v25.0.1364.160
  XP
    Chrome v27.0.1453.110 m
    Chrome v28.0.1500
Mozilla
  Ubuntu (12.04)
    FF 21.0
  XP
    FF 20.0.1
    FF 22
IE
  XP
    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?