Feature/composer support

Fixes problem with executable in #53

To test it out:


 1. Create a new dir and add following `composer.json` file to it

     ```json
    {
        "minimum-stability": "dev",
        "require": {
            "sebastian/phpcpd": "dev-feature/composer-support@dev"
        },
        "repositories": [
            {
                "type": "vcs",
                "url": "https://github.com/Ocramius/phpcpd.git"
            }
        ]
    }
     ```

 2. Run following:
     ```sh
    curl -s https://getcomposer.org/installer | php
    php composer.phar install
     ```

 3. Run the cli tools:
     ```sh
     ./vendor/bin/phpcpd.php vendor/sebastian/phpcpd/src
     ```
     should produce
     ```
     phpcpd 1.4.0-11-g458de68 by Sebastian Bergmann.

     0.00% duplicated lines out of 1597 total lines of code.

     Time: 0 seconds, Memory: 2.25Mb
     ```

Same works without defining the `composer.json` file and using the cloned repository instead.