Add viewcode extension to allow for links to source in API docs
This creates the [source] links in the API documentation sections.  You can read more about this Sphinx extension [here](http://sphinx-doc.org/ext/viewcode.html).  There could be a reason this is NOT already added so let me know.  I find it useful to have this because often times I want to easily jump to the source in the HTML docs.  Of course, I typically just use this to quickly find the file and open it in a real editor.

It might be worth noting this functionality was added in Sphinx 1.0, which was released in July 2010.  I'm not sure what the rules are on backwards compatibility for the documentation build.

I tested this with Sphinx 1.1.3 and it worked without any issues.

It's also worth noting there is no [source] link for things like `pandas.io.parsers.read_csv` which is sort of built dynamically instead of with a traditional `def`.  Maybe there is a way to improve the `viewcode` extension to handle this or put some additional hooks in the documentation itself?  Not sure what the best approach would be in that case or even what options exist.
 This and the fact that almost no user-facing pandas function/method actually does any work other than delegating is one of the reasons I'm not sure about the usefulness of this. That said, it wouldn't hurt anything to have (except the size of the docs, which is probably not a concern). Many refactors ago I got out of the habit of using ?? in ipython to look at the source for pandas because of these same reasons.
 I agree with jseabold, moving to 'someday'
 No problem.  Maybe this would be more important in the future anyway.  Thanks for the explanation.
 Someday PRs make no sense. closing.  Thanks for trying.
