Import Guava utilities

[Justin Richer]

Google's Guava utilities pack many very useful functions and utility classes that would be helpful for building and extending the functionality of Nimbus-JOSE.

[Vladimir Dzhuvinov]

Base64 + Base64URL is the only external utility that is currently used, provided by Apache Commons Codec.

Guava does offer base encoding/decoding, but the com.google.common.io.BaseEncoding class is still marked as @Beta as of v14.

[Justin Richer]

Guava adds a lot more than just the encoding, though. There are lots of useful utilities dealing with objects, collections, strings, and other things that I use on basically every project these days. It's one of the first things I import in Java, but I can understand a desire to minimize external dependencies.