Add user agent support

[carmstrong]

Adds support for user agents -- patterns can be specified with only_agents or except_agents.

[carmstrong]

Anyone have any insight as to why this failed for ruby head only?

[rymai]

Yeah, it cannot find bundler, it's a common issue these days (I never really searched the real cause).

If you rebase your branch on master, I've now allowed ruby-head to fail on TravisCI without failing the entire build.

[carmstrong]

Thanks. Rebased and pushed.

[rymai]

Thanks, sorry I messed up and everything was failing on TravisCI! :P That's fixed now.

Anyways, could you also document the new feature in the README before I merge this PR?

Thanks in advance!

[carmstrong]

Done. I also listed myself as a contributor in the README - not sure if this qualifies me as a contributor. I can certainly remove it if not.

[rymai]

Thanks! Regarding the contributors, I'll actually replace the hardcoded list with a link to https://github.com/tobmatth/rack-ssl-enforcer/graphs/contributors, it'll scale better!