Add support for remapping DNS hosts

I tried to write a Selenium spec but it didn't work for some reason:
```ruby
it 'should remap given DNS hosts' do
  proxy.remap_dns_hosts('foo.newrelic.com' => '173.194.70.113')
  driver.get('foo.newrelic.com') # => NS_ERROR_MALFORMED_URI
end
```
It doesn't remap a DNS hosts though I believe it should.