Added self-update command for phar builds

This contribution added a self-update function and could be invoked by:
 ```$ php phpDocumentor.phar self-update```
For a more complete example of the manifest.json see
```
[
    {
        "name": "phpDocumentor.phar",
        "sha1": "cd8419429e2f131f5f2062f2fc4840bd07bc08a0",
        "url": "http://phpdoc.org/phpDocumentor-2.6.0.phar",
        "version": "2.6.0"
    },
    {
        "name": "phpDocumentor.phar",
        "sha1": "...",
        "url": "http://phpdoc.org/phpDocumentor-2.6.1.phar",
        "version": "2.6.1"
    },
    {
        "name": "phpDocumentor.phar",
        "sha1": "...",
        "url": "http://phpdoc.org/phpDocumentor-2.6.2.phar",
        "version": "2.6.2"
    }
]
```
The manifest.json is the one from the repository defined by 
`const MANIFEST_FILE = 'https://raw.githubusercontent.com/phpDocumentor/phpDocumentor2/develop/manifest.json';`
With help of these manifest is it possible to not only update but to downgrade to a special version as well as for example locked to a major version and/or pre-released allowed versions.

You have to edit the manifest each time a new version is out or an easy batch script found here: http://moquet.net/blog/distributing-php-cli/