Use cookbook-omnifetch to pull remote cookbooks
Add cookbook cache management (WIP)
Integrate policyfile to graph solution with cookbook prefetching

WORK IN PROGRESS, unit tests and refactoring to come
Update unit test to match changed interfaces
Add specs for new CookbookCacheManager behaviors
Add specs for new Policyfile Compiler behaviors
Add specs for CookbookMetadata class
Move CookbookOmnifetch adapter classes to their own files
Add specs for CookbookSpec behavior
Handle version constraints in policyfile
move test scripts out of top level
Rename cookbook_source_overrides -> policyfile_cookbook_specs
Get cookbook-omnifetch from git for now
Merge branch 'prefetch-fixed-cookbooks'