Force conversion to bool when using the ? : operator.

This is needed to be able to overload the bool_ function for gmp
Setup Travis CI

In addition to the various test scenarios stored in .travis.yml:

- make CLI documentation compiler dependent, using pythran-config
  --compiler new flag
- fix a typo in the DEVGUIDE
- split test_numpy in three parts otherwise travis times out. It was too
  big anyway
- fix libs flags
Fix numpy's version of power operator

Test & implementation