[ResolverSpec] Add uses a Podfile requirement even when a previously declared dependency has a different requirement

This was tough to do, since it really adversely affected performance on unresolvable podfiles. Hence the optimization of making sure that a pod's dependencies with the same root name are pinned to the same version in `dependencies_for`

\c @kylef, @alloy

Addresses #3 of https://github.com/CocoaPods/CocoaPods/pull/2637#issuecomment-58584402

Use new Dependency#prerelease? method

[Resolver] Sort dependencies by number of possibilities, improving performance exponentially on large Podfiles

Fixes #2723.

Updates for https://github.com/CocoaPods/Core/pull/131.

Currently supported printer types are:
* `filename.html`: GraphHtmlPrinter
* `filename.txt`: GraphPrinter

For more printers see
https://github.com/ruby-prof/ruby-prof/blob/master/README.rdoc#printers.

Previously the header pattern was basically doing: `*.{.h}`, which has a dot too much,
but just happened to work because we don't actually use `Dir.glob` but implement our
own, which apparently isn't 100% similar.

Refactorings in Preparation to Framework Support

Thanks to @fabiopelosin.