[Resolution] Add rudimentary double backjumping
[Resolution] Add swapping for conflicting `existing` possibilities
[Documentation] Document the new methods
[Conflict] Add requirement trees and the locked requirement

To allow for bundler-style conflict messages
[Documention] Fix spelling error
[1.8.7] Fixes for 1.8.7
[RuboCop] Get create_conflict back to 15 lines
Fixed failing bundler specs
[SpecHelper] Fix test index searching
[Resolution] Remove now-redundant codepaths
[DependencyGraph] Flatten before concatenating
[Documentation] Spelling fix
[Resolution] Remove vestigial all_conflicts attribute
Merge pull request #16 from CocoaPods/double-backjumping

Double backjumping & Forward checking