* Use a single line comment

* Add a separate method to retrieve the Request body

Rather than having a single large routing table, split routing into
separate dispatching classes, similar to a middleware set.

* Remove PHPUnit from composer, require users to install PHPUnit globally
* Upgrade PHPUnit version to 5.4 in Travis CI
* Use TestCase::createMock instead of using TestCase::getMock (deprecated)
* Use TestCase::setExpectedExceptionRegExp instead of using annotations

According to the [intent of ADR][1], the Action part of the triad is
meant to be a completely custom class that provides a boundary between
HTTP and domain logic. The action is aware of the domain and knows how
to direct output through a responder.

This is not how Equip has been treating the domain. Our attempts to
make Domain pseudo-HTTP-aware with Payload and Input has not been very
successful and is probably the least concise bit of the framework.

[1]: http://pmjones.io/adr/

Also includes changes from:

* Remove payload from formatting (#77)
* Restore content negotiation (#78)
* Add documentation for v3 changes (#79)
* Remove payload and input classes (#80)
* Remove content negotiation (#84)

Although ActionHandler deals exclusively with ServerRequestInterface
instead of RequestInterface, ActionInterface used just
RequestInterface. This made it impossible for actions to use the
helpful features that ServerRequestInterface offers.

In particular, this bug actually rendered the example Action code in
the docs incorrect, because the variable $request (of type
RequestInterface) was passed as a parameter to input(), which expected
ServerRequestInterface.