Follow the Freedesktop spec for thumbnail location

According to the Freedesktop specifications for thumbnails [1], the
thumbnails directory should be stored in $XDG_CACHE_HOME/thumbnails, not
in $HOME/.thumbnails. This fixes that

[1]: http://specifications.freedesktop.org/thumbnail-spec/thumbnail-spec-latest.html#DIRECTORY
Merge pull request #30 from EvanPurkhiser/fixup/xdg-thumbnails-dir

Follow the Freedesktop spec for thumbnails location