Add sample encoders

[saxbophone]

Perhaps these could be included in an examples module.

Well-known encoders I'd like to produce examples for:

• Base64
• Base64, URL-safe variant
• Ascii85
• Base85 (revised version of Ascii85 conforming to RFC 1924)
• Z85 (ZeroMQ version of Ascii85)
• Base91
• Base32
• Base16 (maybe too easy?)
• Base36
• Base58

With the exception of the base-85 schemes (which perform some additional kind of run-length encoding on certain output patterns, all of these should be rather trivial to implement as subclasses, and might serve as more helpful documentation and proof-of-concept to potential users.

Regarding the output transformation of the Ascii85 variants, it might be worth holding off until issue #27 is done (the pre-processing and post-processing ideas would be very helpful for such a scheme as Ascii85).