Cleaning up documentation
Followed @astrofrog's suggestion and made these methods take the spectral_cube argument
Only allow yt tests for recent numpy versions
Only need to test 1.7
split out yt tests
fix yt test setup
make world2yt and yt2world private functions
whitespace/pep8
add a link to the yt docs
skip failing test: tests pass!
fix test: yt_version needs to be defined even if yt can't be imported
to_yt returns a YtCube object
pep8 -> package naming convention
fix tests: methods of correct objects
one more excess spectral_factor removed
use zero-indexed coordinates in yt2world/world2yt
allow 0 or 1 indexing in yt2world and back.  Clean their docs a little
use numpy >= 1.8
use StrictVersion for all yt version comparisons
>= not allowed!
Merge branch 'jzuhone_yt_rebase'.  Closes #90