Add various Base encoding functions

[joschi]

This PR adds various Base encoding functions (Base16, Base32, Base64, and some variants).

In fact, it provides the functionality of Guava's BaseEncoding to the processing pipelines.

This can be useful to transport binary data over the wire in a safe-ish format.

Discussion from IRC:

<joschi> jose_: could you elaborate on the use case, please?
<jose_> so briefly i'm at fastly, we're using a lot of graylog internally (and wrote a blog post series on it). i'm doing a demo next week of our WAF product and using graylog as a log analysis platform for the workshop.
<jose_> as part of our logging we export attack data (from the client) base64 encoded to avoid fouling up log parsers (it's CSV records streamed out)
<jose_> rather than trying to escape all possible screwyness
<jose_> so after it arrives and then extracted using a CSV field split, and before it gets stored, i'd like to un-encode it
<jose_> so i can see it natively in the product