Add composer.json

This PR adds support of [Composer](http://getcomposer.org/). It's very useful thing, I can use your library by the following requirement in my `composer.json`:
```json
{
    "require": {
        "azatoth/php-pgettext": "*"
    }
}
```
The only thing you need after merging is registering your package name (`azatoth/php-pgettext`, for instance) [here](https://packagist.org). After that feel free to remove `repositories` block.