Implement static_live_server (Django1.7)

fixes #120

<strike>WIP, not ready for merging
Documentation still need to be addressed</strike>
This is what `test_serve_static_with_staticfiles_app` is doing thanks to `_static_live_server` fixture.
Initially I wrote the test like you described e.g: adding `django.contrib.staticfiles` in `settings.INSTALLED_APPS` in the test, but I face 2 problems.
1 - 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.
2 - When we receive the `live_server` fixture, the live_server thread is already created. It is then too late to modify the settings.

This is why I created `_static_live_server` because I can bind the fixture to the function *and* I can modify the settings before the creation of the LiveServer.

I'll add a comment it the docstring explaining this side effect of the `_static_live_server` fixture.