Add linkcode_resolve function to link to GitHub

Add linkcode_resolve to insert a link to the source code of the
documented element. The end result is a sign that reads `[source]`
at the top right of each function, method and class that is
documented with the numpy-doc standard.

Fixes #128

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For the lazy this is a screenshot, clicking on the link in the top window gets you to the second window:

![screen shot 2014-03-15 at 1 10 16 pm](https://f.cloud.github.com/assets/375307/2429273/9139afb0-ac75-11e3-91d3-449f39076c66.png)