Travis-CI support

Closes #37. 

This must be applied after PR #39, since it uses tox to run the tests. This is to allow multiple Python versions to be tested, and by using tox the tests can be run locally as well as by Travis.

After this PR is merged:
 * Register photo/openphoto-python with Travis-CI
 * Create a test account on trovebox.com (a number of the tests use real API calls)
 * Update the secure environment variables

It's probably easiest for me to do this - let me know what you think.