Made the crawler more robust in general

The crawler now handles errors while downloading images
and retries the download up to 5 times.

Image filenames are retrieved without the size specifier
so we can manually add 't' to retrieve thumbnail images.
Before this fix, the crawler sometimes got images
with size specifiers on them already (like http://i.imgur.com/dFuYFdJh.jpg)
and when it appended a 't' to the end (http://i.imgur.com/dFuYFdJht.jpg)
imgur naturally returned a 404 error.

Added sleep statements between downloads so we don't hammer
imgur servers too much. The crawler now also handles
'imgur over capacity' errors.

Before, the crawler uploaded files with their actual extensions
(like xyz12.png) but for some reason I tried to reference them
all as .jpg files (like xyz12.jpg) which resulted in 404s etc.
I now save the whole filename to the database, and add a 't'
to the end of the filename in the Flask app to create
thumbnail images.

Generally, all of these improvements have resulted in
almost no errors. Pretty nifty, huh?