refactored name to Representor and updated spec setup

@kylef @smizell Please review and merge.

This PR is in preparation for doing some work on this finally:

* renames classes to Representor
* sets up rake tasks
* sets up rspec
* configures gemspec
* updated travis-ci config to test more ruby versions
* sets up code coverage checking

@kylef you can see here how to load rake tasks in Rakefile per your previous question.

closes https://github.com/the-hypermedia-project/representor-ruby/issues/2