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 
For example I think that the following is simpler/more terse. If possible lets try to stick to the existing style of tests here

```Python
def test_projection():
    assert eq(compute(t[['name', 'amount']], x), x[['name', 'amount']])
```