Should add composer support

It would be quite useful to support this library as a composer package. Once this repo includes the composer file, you can use it in your project with the following (bear in mind that the repo name/url should be changed):
```
composer.json
{
    "minimum-stability": "dev",
    "repositories": [{
        "type": "vcs",
        "url": "https://github.com/ignasbernotas/mailersoft.git"
    }],
    "require": {
        "ignasbernotas/mailersoft" : "*"
    },
    "config": {
        "vendor-dir": "core/vendor"
    }
}
```