Subscribe a function to the Parameter

Hi,

To see an example of the subscribe functionality, in different terminals type:

$> roscore
$> rosrun parameter_example parameter_server
$> rosrun parameter_example parameter_example_node
$> rosrun parameter_example rosparam_node set /myint 145

It has a a lot of bugs, for example, if you create two different Parameters in the same node and both of them has a subscribe function it is going to fail because it is going to try to register twice the the ServiceServer where it receives the updates. I'll work on this and things other bugs on the weekend.