Insighttoolkit : Added support for python wrapping ('--python')

Some work was previously done to build ITK with python wrapping (https://github.com/mxcl/homebrew/pull/16918). I used the proposed formula and was able to build ITK with --python, with some minor changes.

An user opened a new issue asking for this feature (https://github.com/Homebrew/homebrew-science/issues/182); I also do need ITK with python wrapping.

This PR was tested with 10.8.3 and 10.7.5 and builds against VTK 5.10.1. You will need to install the latest gccxml, using my PR for gccxml : https://github.com/Homebrew/homebrew-headonly/pull/10 (brew install --HEAD gccxml).

I have added the line *depends_on 'gccxml'* but I am not sure the dependency check will work with an head-only gccxml ? I would some help from a homebrew guru for this part.