Updated calibre to 1.15.0, better icon, added Thumbs.db to .gitignore

Remember that *.ico files are not suitable as package icons, because web browsers don’t pick out the right resolution like the Windows UI does. So *.ico files with multiple embedded images would only unnecessarily increase the file size.
Look at the difference:
calibre.ico: 160 KiB
calibre.png at 128 px: 21.4 KiB

Unfortunately I couldn’t find the changelog of Calibre 1.15.0 in the format like you have in the package, so I didn’t touch it.

By the way, don’t forget to merge this: https://github.com/gep13/ChocolateyAutomaticPackages/pull/1
It’s actually less work to set up automatic packages than it looks like. :)