separating configuration from engines
Create methods to build the various engines.

Since we support only four engines, we can have dedicated methods that
we call and store their values in the attribute 'engines'.

We *should* never store the object in the config, and therefore, everytime
someone need an engine, it should be done via '->engine($name)', and never
via '->settings->{$name}'.

This is still broken.
If there's no logger defined, create one for 'console'.
Don't return undef when a ref is passed to an engine builder.
Get the 'session' from the engines, not the settings.
Inline some code in the `->engine' method and remove a comment.
Get the session from the engines, not the settings.

Also, there's no need to croak if the engine is not set, this is already done
by the ->engine method.
Try to build the session engine correctly.
Fix broken vvalidation.
The engines created by triggers need to be set in ->engines.

All the tests are passing, some refactoring is still in order before being
able to merge this branch.
Remove a comment in the _build_engines method.
Move POD at the end and add empty sections for missing methods.
Remove useless debug in a test.
Move the triggers in the config to proper attributes.

There's two kinds of triggeres: on engines, and on config change.

We add a new attribute 'supported_engines' that contains a list of all the
supported engines by Dancer2.  In addition to that, two private attributes are
also added, one that contains a list of triggers associated to the engines,
and the second one is for the config change.
Clode cleanup for creating the engines.
Drop support for the key 'log' in config.

This was for compatibility reason in Dancer1, but this is not requried
in Dancer2.
Don't iterate over a manually created list of engines.

This list exists in the Config role with the attribute 'supported_engines'.
Remove another useless message in the tests.
Don't iter manually over the list of engines, use the list from the config.
Add some actual documentation for Config.