Google App Engine compatability

After being stung by @Damgaard's claim that I had disappeared from github (I bear him no ill will, I have no excuse for never replying to his requests for updates), I pieced together another branch of praw which should fix the problems with my last pull request on the same topic (https://github.com/praw-dev/praw/pull/239)

I'd like to point out that in the last pull request, @bboe said that the errors I was getting from importing update_checker resulted from a bug in the module itself, which he proceeded to fix. When checking praw for errors today I realized that the bug in question had never actually been fixed. I *think* that this is because of bboe catching "NotImplemented" in line 49 of his bugfix commit (https://github.com/bboe/update_checker/commit/9e6c211210205c1df1e2bcb5dd2be66e6bbd8407), rather than "NotImplementedError" as raised when running the app on GAE. If that's fixed then I think that commits #8973461 and #723c499 can safely be removed from the pull request (although I'm new to git, so I'm not sure how I could do that).

I'd like to also clarify that I have not given this version of praw extensive testing with GAE. I've only tried running it with my bot that only performs a few simple functions, and therefore there could be more GAE compatibility bugs that I have not found.

Other than that, I think this PR addresses the problems with my last one.