Add git support to Cookbookfile
 - Allow a cookbook to be pulled from a git repository
 - Also support 'ref' to give a commit hash to pull
 - Possibly support 'branch' to pull the HEAD from a given branch
Add path support to Cookbookfile
 - Pull cookbook from a path on the local filesystem
Add source support to Cookbookfile
 - Allow unofficial cookbook repositories to be used via 'source' in Cookbookfile
Create local mock API service for specs
 - Instead of mocking out methods, spin up a local mock cookbook repo for tests to run against. This should be able to serve cookbooks as gzips. This might require some monkey patching of knife to get it to talk to the mock server instead of the opscode community site.
Optimize cookbook retrieval
 - Possible: Pester Opscode till they release an API call to retrieve cookbook dependencies
 - Possible: Keep an index of dependencies so we don't have to download cookbook versions we've seen before