Docs improvement

Pull request that includes improvements to the end user docs.

As usual I managed to sneak in some other small changes as well:

* Requirement of `pecl/imagick` bumped to >= 3.0.1
* Added `::1` to the whitelist in the `StatsAccess` event listener
* Added support for multiple configuration files when installed with a custom `composer.json` file
* Made the `Imbo\Resource\ResourceInterface` extend the `Imbo\EventListener\ListenerInterface`
* Changed the `404 Unknown public key` reason phrase to `404 Public key not found`