Tis should now behave more like Bundler

[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.