bump v0.4.3
use json.Number internally, to handle both int64 and float64

This defers the actual parsing of the text value to the numeric type
until you dereference it using the Int(), Int64() or Float64()
receiver methods.

This has the benefit that large integers (>= 1e6) are not marshaled in
floating point exponent notation, and that very large integers (>=
2^53) do not lose precision when marshaled because they were stored
internally as float64s.

This retains backward compatibility with Go 1.0.x because it doesn't
have json.Number.
run travis tests against go1 and go1.1
add MustBool()
Merge pull request #18 from mreiferson/mustbool_18

add MustBool()
bump 0.5.0-alpha
add MustInt64()
Merge pull request #19 from mreiferson/must_int64_19

add MustInt64()
fallback to old type conversions when using json.Number for backwards compat
Merge pull request #20 from mreiferson/backwards_compat_20

backwards compatibility for Set and json.Number
fix tests