[!!!] Rename BaseHttpHandler to AbstractController

This changes the general naming convention for former Handler classes,
that will be called Controllers in the Future.

PHPFramework Apps should also rename their classes accordingly.
The feature that twig template names can get automatically derived from
the controller classname will cease to work if you stick to the
"Handler" naming.
Tests: 100% coverage for EntityMapper
[+FEATURE] Possibility to load plugins

Plugins need to be registered in the settings.ini:

[phpframework_plugins]
VendorName\PackageName\PluginName = 1
[+FEATURE] Signal/Slots
Improve Signal/Slots
Merge branch 'feature/8-signal_slots' into develop
Use class constants as Signal Names
Add CoreSlot to insert PHPFramework hint into HTML output
Make inserting the PHPFramework html comment configurable
Merge branch 'feature/10-framework-ad' into develop
PHPFramework is self-aware about its version now
[!!!] Airbrake must be enabled explicitly

if you want to you use airbrake set:

[airbrake]
enable = true
Add ValidationResult class
Output PHPFramework version in HTML comment
[+FEATURE] Routing to callables
Fix composer.json in README
Composer: PHPUnit is only required on root
Update Twig to 1.16
Switch to a different composer package for airbrake

The current package requires "minimum-stability": "dev" to be loadable
example .gitignore in README