Hooks

I started experimenting with running optional external scripts before and after a project generation, as i suggested in #33. I ignored the initial idea of hooks acting on the context, as this seemed harder to integrate with the current code. I'm thinking the run_hook function could do some dispatching to handle those. The two hooks implemented here could be used to create a virtualenv, initiate a git repo, pip install dependencies...

The way i'm avoiding copying the `hooks` directory is pretty ugly. This suggests that a more standardized way of ignoring some template files/directories could be handy.

I'm still having trouble running tox, so i hope this doesn't break things under 2.6 or 3.*. The whole suite ran fine under 2.7.