Support for non-source packages

Hello,

I just started using your app and I must, say : "Finally someone has done that!!!"

One problem though, for now, when you try to install packages that do not have source distributions (such as [Sphinx](https://pypi.python.org/simple/Sphinx/)) it fails because it changes the URL of the packages from this:

    ../../packages/2.7/S/Sphinx/Sphinx-1.1.2-py2.7.egg

to this

    ../../packages/source/S/Sphinx/Sphinx-1.1.2-py2.7.egg

And therefore gets a 404 not found from pypi.python.org afterwards.

Pull request, coming soon™ Keep up the good work.

--Robin