Fixed file not found bug

Added a test to check the existence of the file without appending
extension to the filename, so now OGXmlConfig and its descendant classes
will load files even if the extension is already given in the filename
Fixed segfault when balls can't find target

OGBall::FindTarget() sets pTargetBall_ to 0, when no target
can be found, which is dereferenced in OGBall::IsCanClimb()
and OGBall::Move().
Fixing bugs of the pipe.

It was possible to open the pipe by catching a falling ball
inside the sensor, then attaching it anywhere, because in
OGContactListener::BeginContact() and OGContactListener::EndContact()
the same method was called and OGSensor::BeginContact() runs after
attaching the ball. The new member in OGUserData won't let sensors
balls_ decrease when doing the same (OGSensor::EndContact() is also
called after attaching) and avoids the pipe to remain open if a ball
enters the sensor attached but leaves unattached.
Setting the isTouching false in OGExitSensor::_EndContact() avoids
the force to keep applied if the ball was pulled away from the pipe.