PROTON-736: Created the UTFString and BinaryString classes for Ruby.

This allows users to explictly call out that a string is either UTF-8 or
a binary string.

For the UTFString type, it validates that the content provided is
actually UTF-8 and raises a RuntimeError if it's not.

Results returned from a Message object are wrapped in the appropriate
class.

This solution is Ruby 1.8, 1.9 and 2.0 compatible.
NO JIRA: Updated the documentation for the Ruby Message class.