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 
Suggest just 

    compute_up(t.sort('missing-field'), x)

Giving the expression a name in this case doesn't seem to enhance readability.