added Gitlab::Git::Blame for git blame feature

lib/gitlab/git/blame.rb

+module Gitlab
+  module Git
+    class Blame
+
+      attr_accessor :repository, :sha, :path
+
+      def initialize(repository, sha, path)
+        @repository, @sha, @path = repository, sha, path
+
+      end
+
+      def each
+        raw_blame = Grit::Blob.blame(repository.repo, sha, path)
+
+        raw_blame.each do |commit, lines|
+          commit = Gitlab::Git::Commit.new(commit)
+          yield(commit, lines)
+        end
+      end
+    end
+  end
+end