Plugin for chroot of authenticated users

This is a small plugin that will set the base directory of the server based on the currently connected user.
For this to be done, I changed some parts of jsDav, which are :
* The tree is now accessed via the handler object (instead of the server object)
* I added the ability to load additional plugins, as the current plugins option key only allows the user to restrict the plugins to use
* I added a call to Path.normalize in calculateUri to prevent security issue when running i.e. http://my_url/.., with or without user chrooting.

There is a test corresponding to this new feature in the test folder.