Add IP field to the Add a Sensor page

[marcocen]

I noticed that when adding a sensor using the web-ui, the sensors IP was always set to whatever IP the PC I was using had at the moment.
This only made sense to me in the case of adding a sensor from the sensor itself, and I don't know how common that use case is.

So anyways, I added an optional IP field to that page so that one can set the sensors IP when adding it. If no IP is supplied, the old behaviour takes over.

I tested:

• Adding a sensor WITH an IP from the webui
• Adding a sensor WITHOUT an IP from the webui
• Adding a sensor WITHOUT an IP using the registration.txt script

In all cases the result was as expected.