Attempting to get some feature parity between yt 2.x and 3.x

Attempting to get better convergence between yt 2.x and 3.0 implementations of to_yt

Adding a yt2world method, cleaning up the documentation a bit

Needed this line

Cleaning up documentation

Followed @astrofrog's suggestion and made these methods take the spectral_cube argument

Replacing pf with ds to reflect yt 3.0

Some unit tests for yt

Adding assert_array_equal for integer ndarrays

Most of these tests pass now, but there's a nasty bug that I need to track down.

These tests won't pass completely until yt 3.0.1 is installed in Anaconda.
Merge commit '875a4c9a26328d9c0833401fa630df5334a7c9ce'

* commit '875a4c9a26328d9c0833401fa630df5334a7c9ce': (242 commits)
  updated CHANGES
  fix a merge mistake in spectral quicklook
  need to pass CASA's units into SpectralCube (also, should check for importerror with aplpy quicklooks)
  fix CASA .image reading: should not assume 3d by default, since usually stored as 4d
  add a quicklook tool for spectral slices
  fix spectral quicklook wcs
  add quick_look documentation
  add a quicklook tool for spectra
  docstring for to_pvextractor
  add a simple to_pvextractor tool
  fix FITS reading bug
  Add Zenodo DOI
  add start_gui keyword
  autoload glue viewer
  need to check component labels, not 'components'
  have to add cubes to the dataset object, not the datacollection
  attempt to accept updates of data collections
  allow a glue application to be specified.  add docstrings
  the glue_app becomes part of the cube now so that subsequent calls add the dataset to an existing session
  first attempt at a to_glue machine.  Required fixing some bugs in __getitem__ but that's no problem
  ...

Conflicts:
	docs/yt_example.rst
	spectral_cube/spectral_cube.py
	spectral_cube/tests/test_spectral_cube.py