Adding Mori as a potential Immutable lib

Hey, this is a bit of a WIP - not sure if it's something you'd want for the project... I decided to extract all of the Immutable functionality into a standalone modules... one targeting Immutable.js, one targeting mori. If `mori:true` is passed in the config options, then it'll assume that was the immutable lib provided to the library.

Got the test suite passing, just thought I'd share to see if this is something you might be interested in.