Add support for remapping DNS hosts

This is the same pull request as https://github.com/jarib/browsermob-proxy-rb/pull/10
The only change is change from '1.2.3.4' to 'plus.google.com' in spec.

For some reason it works with '1.2.3.4', 'google.com', 'plus.google.com' but doesn't work with 'example.com', 'test.example.com', 'plus.example.com'