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
We should treat bad response same way in all cases, so I suggest to move this validation into `initialize` too, and raise an exception if response is not valid.