Create sphinx extension to handle github issue/changeset links
Add a sphinx extension, such that it is easier to link to github issue or changeset in the rst docs.

Idea comes from [pycon 2013](http://pyvideo.org/video/1703/better-documentation-through-automation-creating) and the [code](https://bitbucket.org/dhellmann/sphinxcontrib-bitbucket) already exists for bitbucket. Maybe also for github somewhere?

This way writing e.g

```
:issue:`GH1234` 
```

wil expand to

```
`GH1234 <https://github.com/pydata/pandas/issues/1234>`__
```
