adds postmutliply methods to vector classes

Post-multiplication applies the inverse of a rotation without having to
create any new vector objects or modifying the transformation matrix.
So, v.postmultiply(m) is equal to m.transposed() * v.

This is useful for converting a vector in the global frame to it's
equivalent in a frame described by a certain matrix.

I apologize in advance if I have breached any protocol here, this is my first pull-request ever...