Hacky first/incomplete implementation of extending existing filters.

The renames to middleware still need to happen, but my gut feeling is
that there isn't anything seriously broken with how filters work right
now beyond the configration bits.
Add docs for features that are coming soon.
Add very basic test for DispatcherFactory.
Start test cases for middleware.
Implement matches() and add in progress test cases.
Implement remaining tests for DispatcherFilter.
Mark broken tests as incomplete and remove dead code.
Make priority a configuration value.

Default to the property (which is now protected) but allow users of
a filter to redefine the priority.
Add tests for RoutingFilter.
Remove old filter tests, and update asset dispatcher tests.
Move AssetDispatcher tests to correct test file.
Move CacheDispatcher tests into their own file.
Clean up remaining dispatcher tests.

Clean up the remaining dispatcher tests and try to cover the bulk of the
cases.
Add string filter support.

Like many other places in CakePHP we should allow string classnames for
filters, and let the framework handle construction for simple cases.
Remove additionalParams and update RequestActionTrait tests.

The additionalParams parameter isn't very useful as it simply gets
merged into the request later on. We can easily do that earlier in the
process now.

Fix incorrectlty dispatched array based requestAction() calls. They were
going back through Router::parse() when they did not need to.
Fix failing tests in ControllerTestCase.

Add in knowledge of dispatcher filters. Also workaround Routing being
applied in a filter vs. a Dispatcher method so that the rest of
ControllerTestCase can continue to work.
Add a convention for dispatcher filter names.

Use a Filter suffix for dispatcher filters. I think this will make them
easier to recognize at a glance and mirrors other classname conventions
in CakePHP.
Add missing doc blocks and rename methods to be more consistent.

The feature is no longer called middleware, instead continue using
dispatcher filters. We can't really call this feature middleware as it
acts very differently from middleware in other frameworks.
Allow an array of options to be passed into a dispatcher filter.

When using the string form, it is handy to be able to control the
constructor arguments as well.
Expand on docs around event priorities.

Add some detail around how priorities are handled.