Allow to add a port candidate to a RXTXCommDriver

Normally a NoSuchPortException is raised, if you try to get a port identifier for a not commonly used device name.
This change allow us to add device candidates in the code by using the following lines:

<pre>
final RXTXCommDriver rxtxCommDriver;
rxtxCommDriver = new RXTXCommDriver();
rxtxCommDriver.initialize();
if (rxtxCommDriver.addSpecifiedPort(device, CommPortIdentifier.PORT_SERIAL)) {
    CommPortIdentifier.addPortName(device, CommPortIdentifier.PORT_SERIAL, rxtxCommDriver);
}
</pre>

Dit not know, if this is a "good" solution / workaround.

We could also change the internally hold list of names of common ports, ...
But perhaps this list should not be changeable at runtime.