Created image directory to support new github wiki.

In the wiki article [FAQ Allocate More Memory](https://github.com/OpenRefine/OpenRefine/wiki/Faq-Allocate-More-Memory), an issue attachment from the Google Code Issue Tracker was used.
(http://code.google.com/p/google-refine/issues/detail?id=242)

Github wikis traditionally use a git repo to support images.

Perhaps its best to have an image directory in github as part of the
move, instead of linking to the Issue 242 attachment.