Composer analyser

I thought it might be useful to analyze the required PHP extensions of a PHP project managed by Composer and to include the analyzed deps into the composer.json. You could either do this manually or use the ComposerAnalyser from this pull request to directly output the needed json including the min PHP version.

If you include the deps and try to deploy your project to a server without a required extension, composer:install will fail.

Once you include the ComposerAnalyser in your phpcompatinfo.json, you can use it with:

    phpcompatinfo -v analyser:run . Composer

Sample output:

```
{
    "require": {
        "php": ">= 5.3.0",
        "ext-standard": "*",
        "ext-spl": "*",
        "ext-json": "*",
        "ext-date": "*",
        "ext-dom": "*",
        "ext-SimpleXML": "*",
        "ext-Core": "*",
        "ext-soap": "*",
        "ext-pcre": "*",
        "ext-session": "*",
        "ext-sqlite": "*",
        "ext-PDO": "*",
        "ext-mysqli": "*",
        "ext-gmp": "*",
        "ext-Reflection": "*",
        "ext-hash": "*",
        "ext-xml": "*",
        "ext-iconv": "*"
    }
}
```

The only downside is that it's not telling you which code is actually run by your application.

`JSON_PRETTY_PRINT` is only available since PHP 5.4.0
That breaks minimal requirement for the CompatInfo package that is only 5.3.0

Consider to use a class constant like 

```php
Class ComposerAnalyser extends ExtensionAnalyser

const JSON_PRETTY_PRINT = 128;

// ...
$output->writeln(json_encode($extensions, self::JSON_PRETTY_PRINT));
```