add hitbox
add unit test for hitbox
add comment
added tests
Merge branch 'hitbox' of https://github.com/lijiaqigreat/ateam-tanks into htbxmerge

Conflicts:
	src/HitBox.java
quickfix hitbox, add more test case
Merge branch 'hitbox' of https://github.com/lijiaqigreat/ateam-tanks into htbxmerge

Conflicts:
	src/HitboxUnitTest.java