- fixed ap_param assignment operator
(http://code.google.com/p/arducopter/issues/detail?id=555)
- made cast_to_float const-correct
- changed the return type of the conversion operator from T& to T:
	- the former version
		* favoured unintended changes of _value
		* prevented the option to add code that checks the set value
		* made it hard to make existing code const-correct
- added operator|= and operator&= to eliminate the need to change
	ArduCopter/setup.pde