Docs Makefile additions, compute/numpy tests.

Fixes the problem of capabilities.py failing to create the graph the first time docs are built on a machine. Simple fix. Also, there are some tests and random changes to compute_numpy in here that I added in. Going to remove the other pull request. @mrocklin @cpcloud 
Then we should investigate as to why `x != y`.  Maybe this would be a good time to break out pdb

`py.test` has a pdb flag btw, `py.test  various args --pdb`