WCS at dendrogram compute time, for WCSAxes-viewer integration

This is a few simple changes that allow you to pass a WCS instance to `astrodendro.Dendrogram.compute` and then automatically have the WCS coordinates properly applied when bringing up `Dendrogram.viewer()`. I've tested it on both 2d and 3d data and it seems to work. This addresses Issue #96.

Currently the code trusts the user not to provide an incorrect WCS (i.e., there are no checks for data size or dimensionality).

Thoughts:
* I'll think about whether this change warrants unit tests, and what part of this behavior needs testing.
* Is the WCSAxes import a problematic dependency? I know I always have to install it separately on a new machine whenever I check out astrodendro. Update: I did have to add wcsaxes to the Travis requirements in commit b4325b043b7f1eb4615b950b5f10518c0e69de23
* Now that dendrograms (sometimes) have a .wcs attribute, it would be possible to check for dendrogram.wcs when computing catalogs, if `metadata['wcs']` is not present. Would this be too weird from a UI perspective?
* ~~I bet that this feature needs some mention in the documentation, so I'll find a sensible place to mention it.~~ Documentation addressed in commit a4429b2ea9bf9887ba5ea95930342111793b7e7e


I think wcsaxes should be an optional dependency -- that is, users should still be able to use the viewer even if they don't have wcsaxes installed. Thus, this import statement should be moved inside a function call, so you can still import viewer.py without it