Added the Target queue for the crawler.

1) Fixed the typo in services.GetPiblicKeyForHkid to services.GetPublicKeyForHkid.
2) Created a channel to use as the target queue
3) Created a map to store the already visited target list
4) Created a target type to hold a Hash and a typeString
5) Created ContentFiles to seed the target queue.
6) Created a crawlTarget function to handle the targets from the queue
7) Created helper crawl functions for Crawl Target in handling blobs, lists, commits and tags.
8) Created GetCommitForHcid and GetTagForHcid in services.
Merge branch 'master' of github.com:AaronGoldman/ccfs