Poll camera whenever external trigger is ON (fix #22)

To implement this feature the information about the current state of trigger is propagated from `class Trigger` to `class Camera1394` via new public method `Features::isTriggerPowered()`. To avoid querying of `libdc1394` on every read of a frame the state of trigger is cached in private variable inside `class Trigger`. That variable is updated whenever `set...` or `getExternalTriggerPowerState()` is called.

@jack-oquin, I hope this does not interfere with your vision of how it should be implemented.