Add websms provider

There is an Austrian SMS provider called [websms](https://websms.at/) which I just added support for in SmsSender. I have also added the provider to the list of supported providers and wrote a PHPUnit test (which runs successfully on my machine).

Detailed information on the web API can be found [here](https://websms.at/entwickler/sms-api/api-rest/rest-schnittstellenspezifikation).

Please let me know if there is something that must be improved before merging.

Cheers
Why not using a class constant for that?