Don't use a raw string with \u escapes

Apparently, Python 3.2 doesn't process these in raw strings.
See https://mail.python.org/pipermail/python-list/2012-May/624756.html
and https://mail.python.org/pipermail/python-dev/2007-May/073042.html
Move force_bytes into lib module
Remove @vstoykov's note. Seems like the right place to me (:
Use force_bytes from imagekit.lib in test_cachefiles
Improve logic of contributing ImageSpecFields to Models

- If `source` is defined then register source group immediately
- If `source` is not defined then create a signal handler and attach it to
`class_prepared` signal which will see if there is only one ImageField in 
the given model.

This will fix problems coused in Python 3 about using ImageSpecField
without providing a `source`.