Improve docs

There were some problems with the way the sphinx documentation was configured, in particular the path setting in `doc/conf.py`.  The changes in this pull request fix this, and create "mocks" for numpy and scipy routines so that these packages do not need to be installed for the documentation to build (necessary on readthedocs.org, and a good thing in general).

This branch is based on the 0.6d release, and builds successfully on readthedocs.org (but not on travis.ci, because this release predates that setup).  My plan is to merge this in to master, and then improve the documentation further.