Mutable.Map type validation

[Ta4i]

Seem like there is no deep validation for Mutable.Map values. That is mean that if value is of some complex type then you could use any object for it. I provide unit tests for this issue. You can find them in maksym/provide-unit-tests-for-map-type-checking branch

[amir-arad]

I've looked into it, seems like the validation mechanism deserves some serious attention. I'll get into it after the mobx dust settles.

[idoros]

#48 - just a quick fix for allowPlainValue with {_type}