Add tryParse methods to UnsignedBytes, UnsignedInts, and UnsignedLongs.

[leeyikjiun]

This resolves #1957 by providing the tryParseUnsigned* methods.

[leeyikjiun]

Okay, I will make the changes. Do I have to close this pull request and create a new one?

[lowasser]

I don't think so? I think you can just add more changes to this pull request?

[leeyikjiun]

I can't add the tryParse method for UnsignedBytes as there is no UnsignedByte class to return as a null object.

[lowasser]

Then probably leave it out -- it's not clear what we should do there.

[leeyikjiun]

I've found some further inconsistencies. In UnsignedLongs.parseUnsignedLong, a NumberFormatException is thrown if the radix is not within the min or max radix which is consistent with Java's Long.parseLong. However, in Longs.tryParse, an IllegalArgumentException is thrown instead.

I think the new tryParse methods should throw NumberFormatException as well. But what should we do with the old methods?