Allow Record subclasses

[lukesneeringer]

This request is a replacement for #25, because in trying to set up and test the Travis stuff, I somehow broke the branch.

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.

[lukesneeringer]

@stutrek Behold, automated testing! :-)

[stutrek]

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