added git tagger for rake git:tag_and_push

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
Signed-off-by: Tony Hansmann <thansmann@pivotallabs.com>
updated git:promote_branch

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>
added rake desc for git tasks

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>