Adds static function to call the Plugin::onDependenciesChangedEvent() method

[Potherca]

Fix for issue #4.

This PR expose a new method run. This can be use from a composer.json file like this:

{
    "name": "potherca/phpcodesniffer-composer-installer-example",
    "description": "Example to demonstrate the working of the dealerdirect/phpcodesniffer-composer-installer package.",
    "require": {
        "dealerdirect/phpcodesniffer-composer-installer": "*",
    },
    "scripts": {
        "run-phpcodesniffer-composer-installer": [
            "Dealerdirect\\Composer\\Plugin\\Installers\\PHPCodeSniffer\\Plugin::run"
        ]
    }
}

Beside this feature I have also done some minor code cleanup and added more message for in verbose mode.

[Potherca]

@frenck I would suggest merging #6 and #8 first, I'll pick up any merge-conflicts here afterwards and ping you for a review.

[frenck]

@Potherca Will do 👍

[Potherca]

@frenck Update complete, can be reviewed.

[frenck]

Works fine, but missing some code documentation.

[frenck]

Missing documentation block.
Could you please add this?

[frenck]

Missing documentation block.
Could you please add this?

[Potherca]

Docblocks fixed.

[frenck]

Thx!