Custom per-site settings

This patch provides the ablity to have custom per-site settings. For the ease of use i've put all relevant piwik settings as predefined values into SETTINGS_CHOICES. That way the user can just select from a dropwdown. i'm using setDownloadClasses and addDownloadExtensions at the moment.
One could consider using a TextField instead of CharField for Settings.value because piwik's default value for setDownloadExtensions is already pretty long.