Display triplets and rework downloads

#145: Added DisplayableImageTriplet class which can render a 3-by-3 grid of HDULists using matplotlib.

Also reworked downloads significantly.  It should be simpler now, and will make implementing the download of the 3-by-3 grid easier.

Also added a script in the examples directory showing the current API for parsing an .astrom file, downloading one of the images, doing photometry calculations on it, and then displaying the image.  API still subject to change (especially downloads part).