Hooks cleanup

This incorporates [PR#18](https://github.com/translate/pootle/pull/18) from @chhitz, as well as some additional documentation cleanups and corrections.

There is one substantial change to the hooks implementation, in [commit fb361d8f](https://github.com/dupuy/pootle/commit/fb361d8f8eec25dc9332f62d018fc49d913fbc15) to make sure that if a preupdate hook changes the file to be updated, that the updated filename is passed to the postupdate hook.  This is the behavior that sumo.py (the only instance of a hooks implementation I could find) expected, and which makes the most sense.

I also included some fixes for sumo.py - it is really only of historical interest, as SUMO [stopped using Tiki PHP](http://blog.mozilla.org/sumo/tag/kitsune) more than two years ago, but as the only example of a hooks implementation, it seemed worthwhile to clean it up a bit.

Despite the warning in [bug 2559](http://bugs.locamotion.org/show_bug.cgi?id=2559) that changes in 2.5 might break hooks implementations, this was not the case for sumo.py - and probably not many (any?) other hooks implementations - since it was not depending on actually running VCS operations on the files being committed/updated - it was just running format conversions and making sure the alternate format used in the VCS was also updated/committed (the versioncontrol code that handles that already was updated to deal with the separate VCS directory).