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
code]` at the top right of each function, method and class that is
documented with the numpy-doc standard.

Fixes #128
Merge pull request #166 from ElDeveloper/github-links

Add linkcode_resolve function to link to GitHub