Custom dataType updates

[mmacaula]

Fixes a bug vaguely referenced in #112 where listening to events could be messed up when using setOnce.

It also:

• makes the set call be bound to this, which makes the call more powerful (this is also referenced in Suggestions for custom Datatypes  #112)
• creates an onSet method for custom datatypes, which is designed to allow for initialization of custom datatypes after they pass all their tests (type check, setOnce, etc) but before events are called. Case here is for state type setting up child event bubbling.
• updates for the state compare method to only do compare, child bubbling moved to new onSet method.

Currently does not update docs. #112 has been out there for awhile so I'm not sure how well this PR will be received. However, if I get interest from the core team, I'll update the docs to reflect the new changes. I'll even throw in some documentation for the state datatype (from #142 ). :)

[kamilogorek]

Code looks fine, tests are in place. Thanks!

[mmacaula]

#202 is the new home for this. Closing