ENH: Add doc build

[larsoner]

This should fix AppVeyor and make it build the docs (in noplot mode).

[coveralls]

Coverage remained the same at 79.044% when pulling def2546 on Eric89GXL:appveyor into 419a93b on nipy:master.

[coveralls]

Coverage remained the same at 79.044% when pulling ee43316 on Eric89GXL:appveyor into 419a93b on nipy:master.

[coveralls]

Coverage remained the same at 79.044% when pulling c99d3f3 on Eric89GXL:appveyor into 419a93b on nipy:master.

[coveralls]

Coverage increased (+0.02%) to 79.066% when pulling f2186eb on Eric89GXL:appveyor into 419a93b on nipy:master.

[larsoner]

I'll merge this once the CIs are happy since it fixes AppVeyor

[christianbrodbeck]

Is reverting the mlab backend necessary to get the tests to work? It will make the tests less complete...

[larsoner]

They cause AppVeyor to crash

[christianbrodbeck]

I see... could we make it system-dependent?

[larsoner]

... they also cause windows to open locally during testing, which is less than ideal. The best thing would be to make Mayavi more fully support the test backend.

But I agree it's slightly better to cover more fully, so I'll make a commit to triage for Windows.

[christianbrodbeck]

Yeah I agree windows opening during testing are less than ideal... at least as it is now you can change to the test backend temporarily with one change.

[larsoner]

Okay we're green so I'll squash and merge. But if people feel like looking feel free and we can iterate.