Create script to allow non-collaborators to assign issues

I'd like to start using Huboard more extensively for the weekly standups. Ideally in-progress work should be assigned to someone to track accountability. However, right now, there are two problems with how Github issue assignment works:
- non-collaborators (those not on a team) cannot be assigned tickets, even though they can submit pull requests to solve them. (isaacs/github#100)
- people on a team with "push" access can be assigned, but only by someone with at least "push" access. (isaacs/github#153)

We could write a hubot script to solve this:

### Sample interaction

```
roobot assign <username> <issue#>
roobot assign patcon gh1287
roobot assign patcon www.gittip.com#1287
```

This would add "patcon" to a pre-defined team with *only pull access* to the proper repos. It would then assign that issue to them.

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