Improve/Clean sphinx related code

[TagnumElite]

@niftools/pyffi-reviewer

Overview

Add some of the improvements mentioned in #46 that wasn't implemented before the merge.

• Change BuildDocs from print into raise exception So that it fails the travis build when the docs aren't buildable
• Tell coverage not to cover BuildDoc class The BuildDoc class isn't required to be documented.
• Remove sphinx conf.py test code
• Update sphinx conf.py paths

Fixes Known Issues

None

Documentation

No BuildDoc coverage

Testing

Nothing important changed, all code should be working fine.

Manual

python -m pip uninstall sphinx python setup.py build_docs

Automated

No new errors were created.

Additional Information

Please do not merge this request until I say so please. I wish to see if I can fix some sphinx build errors first. Do tell me what you would like improved/fixed/remove!

[coveralls]

Coverage increased (+0.007%) to 58.38% when pulling 666290df598091a1ea2dafd04e3d876703d5440c on TagnumElite:clean_pyffi into 7f61cc2f40d8896efd4135564bd7e77f903c2315 on niftools:develop.