Add update support.

Add support for updating existing gists in the format
```
gist -u GIST_ID FILE_NAME
```

This closes issue #12.

[Pull request #31](https://github.com/defunkt/gist/pull/31) was originally rejected because it didn't work on private gists, I think since the GitHub API changes this has been resolved, as mine seems to work and does essentially the same thing.