Fix complexity issue in type dependencies gathering

Previous complexity was O(n!), which was a bit too much. New
implementation limits combinatory burst as much as possible.

This fixes issue #102.
Update documentation to reflect recent changes

* Make it clear we use numpy
* Install procedure in the README is simpler
* Add a numpy usecase
Merge branch 'numpy' of github.com:serge-sans-paille/pythran into numpy