Add a new command to list GH pull requests.

The command looks like this:

```
threebean │ .pulls
   zodbot │ threebean: (pulls <username/repo>) -- List the pending pull requests on a github repo.
threebean │ .pulls fedora-infra
   zodbot │ threebean: Must be GitHub repo of the format <username/repo>
threebean │ .pulls fedora-infra/tahrir
   zodbot │ threebean: @hroncok's "Reenable gravatar fallback" https://github.com/fedora-infra/tahrir/pull/209
threebean │ .pulls fedora-watwat/tahrir
   zodbot │ threebean: No such GitHub repository 'fedora-watwat/tahrir'
threebean │ .pulls kennethreitz/requests
   zodbot │ threebean: @dpursehouse's "Linkify Github usernames in authors list" https://github.com/kennethreitz/requests/pull/2140
   zodbot │ threebean: @jamielennox's "Apply environment features to prepared requests" https://github.com/kennethreitz/requests/pull/2129
   zodbot │ threebean: @alekstorm's "Create Request.links_multi and utils function for properly parsing Link headers" https://github.com/kennethreitz/requests/pull/1980
   zodbot │ threebean: @kevinburke's "Connect timeout" https://github.com/kennethreitz/requests/pull/1801
```

It requires that we add an ``github.oauth_token`` to the zodbot config which we
can get from https://github.com/settings/tokens/new
PEP8.
Limit the number of pulls with which we reply.
Merge pull request #17 from fedora-infra/feature/list-prs

Add a new command to list GH pull requests.