Specify #command.
Begin specifying Payload.

Payloads whose body is a hash of zero or more than one mapping are no longer
considered ID maps.
Large-scale Payload refactoring.

This has likely broken things.  I might get around to specifying those things
later to see what's broken but, for now, POSTing and PUTting may fail.
Upgrade sinatra.
Convert specs to RSpec 2.14.7 syntax with Transpec

This conversion is done by Transpec 1.5.1 with the following command:
    transpec spec

* 32 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)
Rubocop the specs.
More nondescript test poking.
Doc/test improvements and some consolidation.

SingleModelObject is now gone, with its responsibilities merged into
ModelObject.

ModelObjects now allow variable handler_targets.  Hopefully all subclass
extensions of the previous behaviour will be removed soon.

Some YARD documentation has been improved.  Yardstick coverage is still low.