Environment Variable Support

This is a Pull Request to add in Redis URL support via Environment Variable to make it behave similarly to Sidekiq and Resque. It'll also enable Timberline to work on services like Heroku where you are provided an environment variable for connections.

Do note, this PR upgrades the test suite to Mini-test as I was unable to determine the Ruby version for the project and could not get the suite working under Ruby 2.0. The suite is passing via `rake test` on Ruby 2.0.0-p353.

Left to-do:

- [ ] Get test suite working correctly around new env var
- [ ] Document new ENV var option
- [x] Allow namespace configuration via ENV