remove buildable artifacts from documentation

The _build directory contains documentation artifacts that should be
generated by a sphinx build and typically should not be committed.
Add an ignore for the _build subdirectories.

The build subdirectories are only used to house generated documentation
and typically shouldn't be committed.
Add the documentation to the sdist tarball.

Currently the documentation is not included in the sdist provided by
PyPi due to this missing Manifest line.
Merge pull request #124 from alunduil/include-docs-in-sdist

Include docs in sdist