Add git http fetcher (smart protocol)

This adds a class to fetch / push git objects through
smart git transfer protocol.
Fix comments and re-org code.
Merge branch 'master' into old_http_fetcher
Rename git_http_fetcher.dart => http_fetcher.dart.
Add missed http_fetcher.dart file.
Add git smart protocol result parser.
minor indent and whitespaces.
Merge pull request #222 from dart-lang/http_fetcher

Add git http fetcher (smart protocol)