added multi-monitor support

Hi,

since I am using a multi-monitor setup with different solutions, I added some basic multi monitor support. You can use your tablet as mouse for a specific screen (use option -s) only. You can also define a rectangular area on the host screen. This comes in very handy when you want to draw stuff - without the fear to mess up your desktop on another screen. Options are:
-s SCREEN_ID
for rectangular frame:
-w width
-h height
-x horizontal-offset
-y vertical-offset

Greetings
joschuck