Set use_mirrors default to "no" for the pip module

Also deprecate `use_mirrors`, if that's ok! I hope I'm not overstepping my bounds; it just seems like a good idea.

Anyway, PyPI moved to a CDN on 2013-05-26, so mirrors are now largely unnecessary and pip removed the functionality in 1.5. More importantly (and why I wrote this request): on 2014-02-15 the mirror directory was taken offline, so mirrors may not work anymore even for pip versions that support them. Somehow one of my machines was able to install things anyway (using pip 1.0), but another one of my machines hung indefinitely when trying to install a new version of a package.