Rewrite to accommodate ActiveSupport 4.1

- Bump ActiveSupport requirement to 4.1
- Update gemspec to recent conventions
- Cleanup deprecated libraries and methods, including using
  Minitest::Test instead of Test::Unit
Set default task to test
Create tmpdir if not exists in order to autrun tests
Merge pull request #9 from jonyamo/rails41

Rewrite to accommodate ActiveSupport 4.1