Some Improvements for Django-recommends Project

Hi @fcurella,

![example](https://f.cloud.github.com/assets/494702/748308/aaa5f8d6-e485-11e2-9b0e-eeca77875848.png) 

Please check the above screenshot when I was using django-recommends in my project, the problem here is that the final result I want to get is a list of my own model objects but not similar objects, if I go through the similar model, there will be a large amount of database call to resolve the object type which may slow down the process. So I propose to allow the `get_similarities_for_object` and `get_recommendations_for_user` method to return a `raw_id (pk)` list of the objects so that we can generate customized query to get our own model objects when we want to use these two method. I have already applied a patch to all the 3 storage and added 2 more tests for the method. [CI passed](https://travis-ci.org/wgx731/django-recommends/builds/8726649) Besides that, I add flake8 check for code style. At last in `.travis.yml` as we have django in `requirement.txt` I am not sure whether `pip install -q $DJANGO` will re-install the correct version of django or not.  Please check out the code and I am really glad if my code can contribute to the django-recommends project.

Thank you very much.
Best Regards,
GaoXiang