Importing tool for admin interface

This pull request provides an importing tool for Django's admin interface. It can assist users and admins in importing new data into CATMAID. Currently, it only supports tile source type one (standard CATMAID file/folder layout). It also expects projects to have a (simple) project file to provide needed information about the projects to be imported. The tool provides a simple way of importing multiple projects at once (and single projects, too, of course) into CATMAID. Details are in the documentation commit.

A custom view is added to the admin interface and an extra module is used for this (adminplus). We could have written this by our own as well, but it would have been be very similar to the module's source (as there are not too many ways to do this) and so I decided to just add this dependency. However, the virtualenv modules need to be updated for this (see updating section in manual).