add a history entry to the header when writing out a FITS file
Merge pull request #134 from keflavich/update_history

Add a history entry to the header when writing out a FITS file
split out ytcube into its own file and make a quick movie rendering
try yt imports.  travis has with/without yt for 2.7 tests
add quick isocontour and docs
add example movie
amend?