Made Tag and Commit fields public then added a parent field to tag

1) Added a parent field to tag
2) Made the fields of Tag and Commit Public
3) Modified cralwer and objects to reflect the Public fields for commit
4) Modified gentagHash to include parent field
5) Modified the tag function in contentObject.go to reflect parent field addition
6) Modified the get.go, post.go, localfileservice.go, fuseIntr.go, ccfs_test.go, benchmark_test.go files to reflect addition of new parent field in Tag and Public fields for commit