Implemented runtime configurable RTS delay

The default delay period when toggling the RTS pin used to be 10ms, which was way too much for my application. I have made this patch that sets the default delay time to one byte, which seems like a good default since the standard requires the bus to be quiet for at least 3,5 bytes before a response can be sent.

I have also added set/get functions so that the delay may be changed from the application without requiring the library to be recompiled.