Support emails in github.user and fix permission exception on getting gists.

* Fetch a correct username: fix requests with wrong username if the github.user value is an e-mail.
* Save gist to a tempfile instead of creating a new file in the root path, which may cause permission exception.
Merge pull request #4 from estum/master

Support emails in github.user and fix permission exception on getting gists.
Merge branch 'master' of https://github.com/hiltmon/Gist.tmbundle