Add Python 2.6 capsule compatibility header

The core_types extension module is not compatible with Python 2.6 due
to the use of capsules, which were introduced in 2.7.

As a workaround a compatibility header that redefines PyCapsule C API
calls in terms of PyCObject is introduced.

See also http://docs.python.org/2/howto/cporting.html
Add .travis.yml