Polygon getPositions

[cozos]

I'm not familiar with JNI, so apologies if this is broken or if there are any missing pieces for adding new wrapper functions

1. Added getPositions() to the Java API, which calls the native c++ impl
2. Added the getPositions_native wrapper to BWMirror 1.2 and 2.1, which takes a Polygon (which extends std::vectorBWAPI::Position) and returns and ArrayList of Positions

Thoughts?