Remove dependency on ActiveSupport (fixes #8)

We only use ActiveSupport for a few core extensions that are easily
swapped out for their slightly-less-readable alternatives. There is also
a single instance of `class_attribute`, but that is easily implemented
as a mixin (included, tested)

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #10 from davidcelis/remove-activesupport

Remove dependency on ActiveSupport (fixes #8)