Switch from require_relative to just require.

This follows the convention of other gems, and solves some require odities which
crop up on Ruby 2.1 onwards.
Adds a Gemfile.
Adds a Rakefile.

* Allows you to run the tests.
* Adds the usual gem tasks (release, package, etc).
Switches test to spec and adds a spec helper.

Calling it 'spec' fits better given the syntax used.
Moves the version to it's own file.

This would allow you to check the version in code, for example.
Adds a top-level file to help import issues.

Possibly a case of common mistake to make; this just allows everything to be
required.
Merge pull request #1 from nickcharlton/gem-path-fixes

Fixes issues with the Gem (paths and missing bits).