Validate docs in tests

[jhamrick]

This clears up the travis config a bit and moves helper scripts into tasks.py so they can be run with invoke. It also adds a check that documentation is properly cleared, and that it produces no errors.

Fixes #127

[coveralls]

Coverage remained the same at 77.89% when pulling 3948db0 on jhamrick:update-docs into c3f6455 on jupyter:master.

[coveralls]

Coverage decreased (-0.1%) to 77.79% when pulling fdaf922 on jhamrick:update-docs into c3f6455 on jupyter:master.

[coveralls]

Coverage decreased (-0.1%) to 77.79% when pulling b8d4ffd on jhamrick:update-docs into c3f6455 on jupyter:master.

[coveralls]

Coverage decreased (-0.1%) to 77.79% when pulling c16824e on jhamrick:update-docs into c3f6455 on jupyter:master.

[coveralls]

Coverage decreased (-0.1%) to 77.79% when pulling 3d951d4 on jhamrick:update-docs into c3f6455 on jupyter:master.