Add Makefile for easier building and testing

Makefile

+.PHONY: test test_ruby test_ruby_rubocop test_ruby_rspec test_go test_go_format test_go_test
+
+validate: verify test
+
+verify: verify_ruby verify_golang
+
+verify_ruby:
+	bundle exec rubocop
+
+verify_golang:
+	support/go-format check
+
+test: test_ruby test_golang
+
+test_ruby:
+	# bin/gitlab-shell must exist and needs to be the Ruby version for
+	# rspec to be able to test.
+	cp bin/gitlab-shell-ruby bin/gitlab-shell
+	bundle exec rspec --color --tag '~go' --format d spec
+	rm -f bin/gitlab-shell
+
+test_golang:
+	support/go-test
+
+setup: compile
+build: compile
+compile:
+	bin/install
+	bin/compile
+
+check:
+	bin/check
+
+clean:
+	rm -f bin/gitlab-shell