Implement static_live_server (Django1.7)

fixes #120

<strike>WIP, not ready for merging
Documentation still need to be addressed</strike>
> the `live_server` fixture is bound to the session so it might have been created long time ago already, thus modifying the settings is too late.

You would have to use the `testdir.runpytest()` approach, where pytest-django would be run with a single test only, and therefore the `live_server` fixture would be untouched.