ExtensionInstaller

Add an ExtensionInstaller for installing PHP C extensions from source

This PR simply copies the compiled extensions into `vendor/ext`. You can then set the `extension_dir` directive in php.ini to point to that.

It makes a few assumptions about the extensions' package:

* user has `phpize` and `make` in the `PATH`
* you can cd into the directory and run `phpize && ./configure && make`
* the compiled extensions go into `modules` (afaik standard for php extensions)

## Sample composer.json

    {
        "repositories": [
            {
                "type": "package",
                "package": {
                    "name": "mkoppanen/php-zmq",
                    "type": "extension",
                    "version": "1.0.0",
                    "source": {
                        "url": "https://github.com/mkoppanen/php-zmq",
                        "type": "git",
                        "reference": "master"
                    }
                }
            },
            {
                "packagist": false
            }
        ],
        "require": {
            "mkoppanen/php-zmq": "1.0.0"
        }

## TODO

* windows support
* docs
* discuss php.ini generation