use MutatingArray for returned arrays

A MutatingArray has the special property that whenever it has new
values being added to it, it will run those values through its internal
array of `filters`.
need to return a MutatingArray from `#deserializeArray`
build with MutatingArray
make MutatingArray a mixin
remove addFilter in lieu of observing `filters`
Merge pull request #12 from jclem/mutating-array

use MutatingArray for returned arrays