Modifications to solve some compile issues
Modifications to solve some compile issues

- Added include guards in header file [stlastar.h]
- Added default constructor (MSVC wouldn't compile without it) [stlastar.h]

majinjing3/astar-algorithm-cpp,justinhj/astar-algorithm-cpp,gerram/astar-algorithm-cpp,justinhj/astar-algorithm-cpp,majinjing3/astar-algorithm-cpp,gerram/astar-algorithm-cpp