Gist UI improvements

Not especially pretty, but how about something like this?

It now pulls your access token from NSUserDefaults rather than the git config.  Possibly `-[PBWebHistoryController getConfig:]` could be removed now, nothing else is using it.

It could be argued that the access token ought to be stored in keychain rather than NSUserDefaults, though the current behaviour is no worse than storing it in .gitconfig ...

![screen shot 2013-07-04 at 10 06 01](https://f.cloud.github.com/assets/2377/748437/faefb554-e488-11e2-8371-4e5cbeecc444.png)