Rename the executable_path option to node_path

[nesk]

The actual option name is confusing.

This:

https://github.com/nesk/rialto/blob/e51cd3106cdf046a11fd18cef9a48e7f5bac71d3/src/ProcessSupervisor.php#L41-L42

should become:

// Node's executable path 
'node_path' => 'node', 

Note: all occurences of executable_path should be replaced in the documentation.