Asking Travis CI to build docs

Trying to get Travis to run sphinx and see if it gets any errors.
Removed empty inversion.heuristic

It was causing warnings in the doc build. I'll turn on doc building in
Travis CI and make the warnings into errors. So this had to go.
Doc corrections in mesher

Doctests in mesher.Tesseroid were unindented causing an error in the
doc build.
Fixed Travis config for doc building

Added rule to clean the build binaries before building the docs and make
warnings into errors
Bugfix in Travis CI config

Forgot a -d in sphinx-build options. Have to build inplace for nosetests
to work.
Rearranged Travis config rules

Putting dependency install and doc building in `before_install` rule so
that I don't have to clean the build before running sphinx. Was having a
problem with find + rm
Forgot to backout of doc folder in Travis

Added `cd ..` to before_install rule
Merge pull request #64 from leouieda/travis

Asking Travis CI to build docs