Add NSCoding support to AFOAuth1Token

[gspiers]

Makes it very easy to persist the token. We are using it to encode to NSData and then store in the Keychain.

[mattt]

Thanks so much for the patch, @gspiers. This is a great feature to have. I cherry-picked eb619ce, and replaced the string constants with literals in 3e02873 (in any case other than NSCoding, I'd use constants, but here, it's just a lot cleaner, IMO).

I also went ahead and made AFOAuth1Client NSCoding compliant as well (since AFHTTPClient is).

...which led me to adding NSCopying for each. What?

As for changing the signature of the callback blocks, this would be a significant API change that I have an alternative idea for, which will be discussed in #39. Cheers!

[gspiers]

Thanks @mattt! Really appreciate the detailed response and #39 is exactly what I am after.