Add Orderable concern, reference implementation.

- Current spec status: 336 examples, 0 failures in 1.47 seconds.
Merge branch 'master' into feature-ordering
Refactor Orderable spec shared examples.

Current spec status: 336 examples, 0 failures in 1.1 seconds.
Add :as, :order_nil? options to Orderable.

Current spec status: 352 examples, 0 failures in 1.16 seconds.
Add :descending option to Orderable.

Current spec status: 361 examples, 0 failures in 1.15 seconds.
Refactor sort criteria into metadata.

Current spec status: 372 examples, 0 failures in 1.24 seconds.
Refactor Orderable :order_nil? to :filter

Current spec status: 377 examples, 0 failures in 1.23 seconds.
Refactor Orderable to accept arbitrary sort commands.

Current spec status: 373 examples, 0 failures in 1.21 seconds.
Refactor to remove unnecessary scaffolding.

Current spec status: 367 examples, 0 failures in 1.18 seconds.
Implement collection ::reorder helper.

Current spec status: 369 examples, 0 failures in 1.22 seconds.
Update documentation.

Current spec status: 369 examples, 0 failures in 1.52 seconds.
Merge pull request #1 from sleepingkingstudios/feature-ordering

Add Orderable concern.