Simplify Grid to be just a data structrue with no knowledge of Level
Change current_ to unsigned to fix signed/unsigned compare warning
Use C string for compatibility with old version of std::ifstream map()
Change std::abs (for floats) to abs (for ints) in integer math
Convert floating-point positions to integers for drawing
Remove unused header cstdint to fix warning
Add commented Makefile hack for nullptr on GCC 4.4
Merge pull request #8 from doug1/grid

Simplify Grid to be just a data structure with no knowledge of Level
Merge pull request #9 from doug1/warnings

Fixes for warnings when building on CentOS