New feature - Option to edit team name and/or permissions

I realized after creating 24 teams that I'd left out the `--perm` option after creating them. By default, the teams were given `pull` access only, but I wanted to give them `push` access. 

This new feature allows you to retroactively change permissions, and even the team name. Reference: 
> http://developer.github.com/v3/orgs/teams/#edit-team

Usage:
`python orgman.py org update --team oldTeamName [--name newName] [--perm newPerm {'push','pull','admin'}]`

I fully tested the feature on a test organization.