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


Cool. 
Things to consider:
* Should we raise an exception at `dendrogram.compute`, rather than at `dendrogram.view()`, if someone tries to supply `wcs` to `compute` and doesn't have `wcsaxes` installed? If not, the dendrogram will awkwardly be carrying around a useless `self.wcs`, and the user won't know it's useless until she tries to call `d.view()` -- and will, at this point, be completely unable to use the viewer until `wcsaxes` is installed.
* If, in the future, the `wcs` object is preserved when a dendrogram is saved to file: what happens if the user who saves the dendrogram provides a `wcs`, but the user who loads it from file doesn't have `wcsaxes`?
* Related to the above two points: if `wcsaxes` isn't installed, would it be better just to raise a warning and then create a "regular" axes, i.e. the same behavior as if no `wcs` was provided? Instead of raise an exception.