add coverity badge, issue #146 [skip ci])
Revert "upgrade cobertura-maven-plugin to 2.6"

2.6 is not compatible junit4/robolectric

This reverts commit 215e10ed5716e7255490a9c36abe768e28a9743e.
check if shortest path will cross min or max, issue #147
add a few more tests, issue #147
fix description
CID 60865 : fix Coverity issue : dereference null return value
test steps into each quadrant, add constants, issue #147
test negative steps into each quadrant, issue #147