Python 3.3

This PR contains changes that allow jedi to work on python 3.3. All tests are passing on my local machine and I updated also travis.yml to run CI on version 3.3.

Changes that I had to do:

* Decoupled the import resolving logic from the imp module defininf a new helper in _compatibilty that choose to use imp or importlib depending on the environment.
* Changed hash generation for cache to use hashlib rather than the builtin hash function as the latter was generating different hashes for the same string in different executions of the pyhon 3.3 interpreter.
* Small changes to enhance compatibility.