Switched from home grown image tool to Picasso

Moved from the home-grown image downloading and caching code to
Square's Picasso library (issue #56). Now we have the advantage of
stuff like memory caching and lots of little extras if we should need
them.

We still may want to set up some method to prune our image cache when
the user logs out.
Add Placeholder and Error Images

Make some simple placeholder and error image files for Picasso to use.
This prevents a previous cover from "flashing" when scrolling down. It
also makes for a nice placeholder as the application downloads the
images.

Additionally, include the source files that created the placeholder.
Updated Picasso to 2.0.1
Merge branch 'working' into picasso

Update feature branch to reflect the changes made in the main
working branch.
Remove Unused ImageDownloader Section

Clear the commented-out ImageDownloader section. Picasso keeps the
cache clean and expires based on the HTTP headers.
Add Placeholder Images to Manga Detail View

Missed this in my earlier changes. Added the placeholder images when
loading covers in the manga detail view.
Merge branch 'picasso' into working

The Picasso library is now stable enough to replace the hacky
ImageDownloader we were using.