Disable pre/post save signals as these will no longer be required once we switch to the filer fork that enforces logical == actual urls.
Update filertags to work with the new filer fork (that supports logical == actual urls):

* trigger css rewrting signals only if the logical == actual url feature is not enabled
* if logical == actual urls filertags uses a faster implementation that performs a single query
* remove url caching as it might cause some issues with the domain that filerfile returns.
Merge pull request #6 from pbs/disable_signals

Disable pre/post save signals as these will no longer be required.