Fixes 'CherryPyLiveServerPlugin: Settings cannot be imported' warning.

If DJANGO_SETTINGS_MODULE environment variable is not defined,
nose raises warning:

    /home/art/myproject/env/local/lib/python2.7/site-packages/nose/plugins/manager.py:395: RuntimeWarning: Unable to load plugin cherrypyliveserver = nosedjango.plugins.cherrypy_plugin:CherryPyLiveServerPlugin: Settings cannot be imported, because environment variable DJANGO_SETTINGS_MODULE is undefined.
    RuntimeWarning)

This happens because cherrypy plugin tries to load django's internals when it loads itself.
To fix the problem, I've moved import of `AdminMediaHandler` into the `start_server` method.