Add tests covering IPv6

As per #182, this adds support for running Webmachine on IPv6 addresses. Notable changes:

- Adapters get now started for every test (before :each, instead of before :all)
- Webrick takes 2 seconds to shutdown, thus I added a timeout that just kills the server thread

All adapters have trouble building URIs with IPv6 addresses - I fixed HTTPkit and Reel so far.