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 
OK, perhaps we shouldn't enforce what Blaze does in this situation.  I suggest that we only test the record-array case (what `x` is above)