Add Makefile to Push Gateway.

This Makefile was copied and adapted from the haproxy_exporter and
allows building, testing, as well as releasing and uploading.
Merge pull request #3 from prometheus/feature/makefile

Add Makefile to Push Gateway.
Fix makefile, update .gitignore.
Merge pull request #4 from prometheus/fix/makefile

Fix makefile, update .gitignore.
Merge branch 'master' into feature/webui
Tweak Makefile, build info, readme.