Bugfix zmq returns string

Running the example client-reverse, on the current master the following error occurs:

```
You can provide an argument to reverse, using abc now.

[DEBUG] Sending request to worker "reverse" with argument "abc"...
PHP Fatal error:  Uncaught exception 'RuntimeException' with message 'Unable to decode: .' in /home/reen/PhpstormProjects/webshop-application/vendor/alphacomm/alpharpc/src/AlphaRPC/Common/Serialization/PhpSerializer.php:31
Stack trace:
#0 /home/reen/PhpstormProjects/webshop-application/vendor/alphacomm/alpharpc/src/AlphaRPC/Client/Client.php(428): AlphaRPC\Common\Serialization\PhpSerializer->unserialize('')
#1 /home/reen/PhpstormProjects/webshop-application/vendor/alphacomm/alpharpc/src/AlphaRPC/Client/Client.php(377): AlphaRPC\Client\Client->handleFetchResponse(Object(AlphaRPC\Client\Protocol\ExecuteResponse), Object(AlphaRPC\Client\Request))
#2 /home/reen/PhpstormProjects/webshop-application/vendor/alphacomm/alpharpc/src/AlphaRPC/Client/Client.php(251): AlphaRPC\Client\Client->sendExecuteRequest('tcp://127.0.0.1...', Object(AlphaRPC\Client\Request), NULL)
#3 /home/reen/PhpstormProjects/webshop-application/vendor/alphacomm/alpharpc/src/AlphaRPC/Client/Client.php(281): AlphaRPC\Client\Client->startRequest('reverse', Array, NUL in /home/reen/PhpstormProjects/webshop-application/vendor/alphacomm/alpharpc/src/AlphaRPC/Common/Serialization/PhpSerializer.php on line 31
```

This bug was introduced in #30 and fixed in 6c2b2bc.

The other commits are minor cs/doc fixes.