add feature for search closest path

[Tarzeron]

Fixes #3

[fgenesis]

Two things:

• closestBestDistance should be ScoreType.
• M_MAX_UNSIGNED is not a standard symbol, might want to use unsigned(-1) or UINT_MAX instead. But since ScoreType can also be float, there should be a MAX_DISTANCE constant depending on the type.

If you want to do changes go ahead, otherwise i'll merge this in a bit and fix it up in the process. Whatever you prefer. And thanks for this!

[Tarzeron]

hi i have corrected your requirements and added some more optimizations

[luodaoyi]

Great