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 decribed by a certian matrix.
creates some tests for postmultiplication
Beefs up documentation comments, tests, removes vector4 version of postmultiply
Merge pull request #37 from pseudopeach/master

adds postmutliply methods to vector classes