Add 'Show Images Folder' option to the file menu

This patch adds a menu item to the 'File' menu that opens the file manager to the folder that stores pyMeme's images.  This makes it easy to curate the collection of images from which the memes are made.

![watch-out-guys-we-got-a-badass-over-here](https://f.cloud.github.com/assets/1383181/1046798/7f984ebe-104a-11e3-86b8-4977b865c258.jpg)