Clean up gemspec.

* Removed dependency on Jeweler
* Cleaned up gemspec
* Removed extraneous files (TODO, VERSION.yml)

For those who want to use bundler, the Gemfile merely relies on the gemspec now.

Fixes GH:11 and GH:12
`require_relative` so tests can run individually

This requires ruby 1.9.x, if this causes any issues for people, we can include the require_relative gem.
Use require_relative for internal files.

There's no need for computing relative paths, let `require_relative` handle it for us.