Don't encode bools, ints, and doubles as strings

[jdreaver]

I was always under the impression that Bool, Int, and Double must be encoded as JSON strings in CloudFormation. Apparently that is not the case, see #85 (comment).

We could probably change the ToJSON definitions of everything to not use Bool', Integer', and Double'. Then, in the FromJSON instances for those aforementioned types, we can try to decode as a JSON number/bool, and if that fails try to decode from a JSON string.

We'll also have to test this on a lot of nontrivial templates. As long as the JSON spec document is accurate with types (hah! wouldn't that be nice...) there shouldn't be issues.