Allow Record subclasses

[lukesneeringer]

This change allows subclasses of a Record type to satisfy checks within typed-immutable.
Currently, in typed-immutable, if you have a situation such as:

var A = Record({foo: Number}); class B extends A {}

And another composite record:

var MyRecord = Record({a: A});

If you send a B instance, it will not throw TypeError, but the instance is typecast back to A.

This change fixes that; it remains a B instance.

[stutrek]

I like it! let's wait until we figure out the automated tests

[lukesneeringer]

I mucked up git somehow. I will open a new PR.