XML Requests blockNumbers, unblockNumbers, getBlockedNumbers

XML Requests blockNumbers, unblockNumbers & getBlockedNumbers implemented.

Please provide me with feedback. I'm very open for suggestions on how to improve it.

Things to note: 

1) I moved the open_server_connection and parts from post_data_to_server from sms_interface.rb to a new file called message_media.rb and added a function for the XML API (post_xml) that shares the open_server_connection method.

2) Tests for ParseXmlSmsInterface will follow

3) I added a dependency for nokogiri which enables me to parse the XML responses easily.

4) All code added by me passes rubocop best practice recommendations.

5) The tests don't need to communicate with MessageMedia. The responses are mocked. If you want to run a test against their system change Rumeme.configuration.testing to true in sms_interface_spec.rb and add username and password as environment variables MEME_USERNAME and MEME_PASSWORD.

Hartwig
You're absolutely right that it can be removed.

I thought that it would do no harm and that there may be some use case that I hadn't thought of that someone using the gem could implement if they had access to the raw data.

But I've got no problem removing it.

@scarver2 what's your opinion on this?