Add support for parsing json.Number to uint64

[onlyice]

json.Number can represent a number in range [2^63 + 1, 2^64), which is suitable for an uint64 value but overflow for an int64 value. When parsing json.Number to uint64, current implementation can't handle number in this range.

Since json.Number is basically string, we can parse string to uint64 directly. I've also add some unit tests.

[mitchellh]

There are a couple test failures that look trivial to fix. If you can fix those up I'd be happy to merge, your impl looks good to me.

[onlyice]

Sure, I've submitted the fix. Please take a look.

[mitchellh]

Thank you! I'll cut a new tag soon, maybe tmrw morning.