feature request: added methods to set min&max of a slider

I added two convenient methods to set the minimum and maximum of a ofxSlider. 

At the moment it is not possible to change min/max after setup. Only via deriving ofxSlider and adding the methods in the derived class. getParameter() is of little help here because it only returns a ofAbstractParameter, which also does not support setting minimum or maximum.

Tested on Ubuntu.