Refresh gem setup and tests

- Ruby 2.1.0 and AR 4+ is now mainstream
- Drop support for Ruby 1.8 (gem should still work, but no tests for that)
- Minor README testing section update
- Remove Jeweler in favor of plain gemfile and Bundler gem tasks
- Minor code cleanup
Merge pull request #71 from spectator/refresh_gem

Refresh gem setup and tests