failed type assertion returns correct zero value

Just checked out your simple json package and I like it very much.

The methods Map, Array, Bool, String, Int and Float use type assertion, but when the type assertion fails, a hardcoded value is returned (like -1 for int and float).

I recommend returning the actual zero value the language specifies instead of hardcoding a certain value. Zero value for int and float are 0 not -1. see http://play.golang.org/p/Q3EsCLYYT3 for an example

i already altered the methods accordingly and `go test` succeeds.

also: *please run `go fmt`*