Watermark / Overlay processor (see issue #113)

Adding processors for placing watermarks in images:

- TextWatermark for typing text directly
- ImageWatermark to use another image as overlay
- ReverseWatermark, same as ImageWatermark but with base and overlay switching places

The TextWatermark may seem superfluous but it might be extremely handy if used directly (outside specs, invoked directly over an imagefield) with text dynamically generated from contextual data (eg: tagging pictures with their URLs, dates or author names).

You can find examples of use and results [here](http://miguelventura.pt/django-imagekit-watermarks.html).