Modifying PathPoint class:
	1: Remove unnecessary code.
	2: get() and getNext() now return -2 as error occurred.
	3: getNext() will return -1 if it reaches the end of vector.
	4: Change the type of pri. var. _currentIndex from size_t to int.
Change:
	1. Pause navigation won't skip current path point now.
	2. Navlog will be open() for the first time, and close() when it
	   reaches the final target.