Give priority to certain extensions

While using PIL's EXTENSION list for our format-to-extension is cool,
simply iterating to get an appropriate extension isn't always desirable
since one of the more obscure extensions may occur first. This commit
allows us to give precedence to certain extensions, but still honors
PIL's EXTENSION dict.

Related: jdriscoll/django-imagekit#186
Add default extensions test
Bump version to 0.1.2