/proxy/:port/hosts and tests.

https://github.com/webmetrics/browsermob-proxy#rest-api describes an
API for overriding DNS.

Though the API is described in the browsermob-proxy documentation,
it wasn't in browsermob-proxy beta 6.

To use this before an official browsermob-proxy beta 7, you will have
to build browsermob-proxy:

git clone https://github.com/webmetrics/browsermob-proxy
mvn -Prelease install

Then look for the zip file in the target dir

If browsermob-proxy throws IllegalAccessErrors when requesting
mapped hosts, this provides a workaround:

https://github.com/webmetrics/browsermob-proxy/pull/74
Fix blacklist and whitelist
Update Client according to new requests' interface

`json` is not a property anymore, it's a method
Empty strict caused json decoding failure (None is returned now for empty
response)
Update setting selenium proxy

Format was incorrect: it should be hostname:port
Setting another types proxy is supported in new method: `selenium_proxies`