Remove PIL dependecies

Based on a [previous conversation](https://github.com/vandersonmota/model_mommy/pull/59#issuecomment-11286778) with @vandersonmota I've opened this pull request with a primary approach just to express what i was think. The only point that annoys me is the tests. Maybe we can skip then explicity [using unittest](http://docs.python.org/dev/library/unittest.html#skipping-tests-and-expected-failures).