This is a crap program and I hate it.

This was throwing an exception in the case where no value was set.

- images no longer print out over and over again (I indented something
  wrong, go figure)
- output contains thumbnail size data
- existing thumbnail sizes are checked (this probably has some overhead,
  but hey, correctness, right?)

- Switched up the CLI a bit
- Simplified the class a bit
- Added height & width to img tags

Needed to create self.image_list in the constructor.

...which don't have EXIF data.  I'm definitely not the first person to hit
this.  See also:  https://github.com/saimn/sigal/issues/11

Hey now it's longer and harder to follow!

- can now spit out a JSON array of thumb & image
- doesn't die when -x and -y differ
- sorta correctly documents effect of -x and -y

This is to enable changes over on p1k3.com where I'm using a JSON list of
images to generate the random header image.  I wanted this because I'm
not using Flickr on the site any more, because fuck Verizon (who, in case
you had missed this iteration of the dismal tide, just bought the corpse
of Yahoo!).