add a command for listing all open pull requests

You can run:

`hub pulls`

And get output like:

```
jm3 - Add correct hub alias for fish shell (fixes #248)
https://github.com/defunkt/hub/pull/350

pchw - add pull-request to open-browser option
https://github.com/defunkt/hub/pull/344

mmozuras - Figure out default branch for the project instead of just using master
https://github.com/defunkt/hub/pull/326

timmow - Add info on using with Github Enterprise
https://github.com/defunkt/hub/pull/297

jianli - browse -b flag
https://github.com/defunkt/hub/pull/258
```