add travis support


You can see travis in action at https://travis-ci.org/quiver/python-snappy

Some notes :
- Python 3.0 and 3.1 is not available on travis
- cffi requires pypy 2.0>=, which is not available on travis, so pypy test fails
  (http://cffi.readthedocs.org/en/latest/#installation-and-status)
  If there's a workaround, please let me know.
- nose is installed by default on travis, so "pip install nose" step is not included.