Add a tox environment for building documentation

[lvh]

This closes #131.

[reaperhulk]

Woud you mind rebasing this now lvh? :)

[hynek]

couple notes:

• the _static thing is not necessary, we’ve killed it off in Fix error message about non-existent _static path #231.
• we don’t need to pin Sphinx to 1.2.x, do we?
• we have py34 now

[lvh]

@hynek What would you prefer to see instead re: Sphinx?

[hynek]

I wouldn’t pin it at all; pyca/cryptography doesn’t either.

[lvh]

I'll rebase this once the other two things come through. Should be easier to just start from scratch though

[hynek]

agreed

[lvh]

Note to self, do this one once #127 is merged, but do it by closing this one + copypasta

[hynek]

note to @lvh, #127 is merged ;)