Add support for multichannel images

[IgorX2]

By default, DIGITS converts RGBA images to RGB. It would be useful to add support for 4-channel PNG (or maybe TIFF) images. Most of the code for 4-channel PNG images is already there.

I tried with modifications to the image.py in utils, but the modifications need to be done in other places too. I can add this functionality if someone points me to the files that need to be modified for this to be done as simply as possible.

[lukeyeager]

Supporting 4-channel image data might not take too much work. One complication is that we'd need to disable JPEG encoding when saving to LMDBs. Unfortunately, I don't have time to scope it all out right now.

FYI, eventually we want to accept many more data types with something like #197 (comment).

[lukeyeager]

@jmancewicz you were looking into this last week, right? Any updates?

[jmancewicz]

The change to image_to_array to also accept 4 channels was added with #1188