RF: allow ipython notebook filenames with spaces

sphinx directives need `final_argument_whitespace=True` in order for the
final argument to a directive to contain whitespace. So, we need to set
this to allow spaces in the ipython notebook filename.  Spaces are
fairly common in notebook filenames because the filename doubles as the
title.
Enable relative paths for notebooks

Specifically, this enables usage like:

    .. notebook:: ../notebooks/blah.ipynb

for notebooks that are in a separate directory relative to one's
rst source.
RF: raise informative error for spaces in path

Space in the notebook file name seem to be hard to deal with, error for
now.
Merge branch 'master' of https://github.com/embray/RunNotebook into embray-master
Merge branch 'allow-whitespace' of https://github.com/matthew-brett/RunNotebook into embray-master

Conflicts:
	notebook_sphinxext.py