PVectors return themselves when modified

Most other APIs that have vectors use methods similar to those used with Java Strings in which methods that modify the object return that object. The branches are auto-merge-able.

For example:
http://jd.bukkit.org/rb/doxygen/d0/d19/classorg_1_1bukkit_1_1util_1_1Vector.html

In effect, all of the modified methods now "return this".