Multi directory

This is how we have implemented multi-directory support for integration into wakari.  
there are two new URLs 
/notebook_direct  which opens notebooks given their full path
and
/notebook_relative which opens notebooks with a path relative to the directory where the server was started

I think there are still some bugs in this code.  filenbmanager reimplements a lot of the nbmanager code because I found the inheritance to be confusing.  Tell me what you think.

Note: I work for Continuum Analytics on the Wakari team.