Tox support

This pull-request add tox configuration to test against all supported Python versions.

HTTPretty is upgraded to 0.6.5 to support requests 2.0 (see https://github.com/gabrielfalcao/HTTPretty/pull/101).
HTTPretty 0.7.0 is broken with Python 3 (see https://github.com/gabrielfalcao/HTTPretty/issues/119).

This allow to see failures locally without the need to wait for Travis CI to run tests.