Adding wcs as an input initializing argument.
Renamed ax1->ax_image, added WCSaxes support when dendrogram.wcs is defined.
Renamed ax2->ax_dendrogram.

ax1 and ax2 seemed unclear to me so this should help the code naming scheme be clearer going forward.
Updated docstring for `wcs` kwarg.

WCS is used for the viewer.
Added wcsaxes to Travis requirements. (I hope this works.)
Added screenshot and snippet about WCS integration into the docs.
Install information for wcsaxes added to docs
Raise an error in Viewer if `wcsaxes` not installed but `wcs` given.
Without wcsaxes, we fallback onto regular axes behavior.

I introduced the variable __wcsaxes_imported, which may be an undesirable style move.
Remove -q from .travis.yml pip install astropy (re: @astrofrog)
Revert "Remove -q from .travis.yml pip install astropy (re: @astrofrog)"

This reverts commit 40301128f4a99b3eca5fc85da34a5eef3ee1ef43.
Merge pull request #126 from tomr-stargazer/wcs_integration

WCS at dendrogram compute time, for WCSAxes-viewer integration