Implemented as specified in RFC 4648 (http://tools.ietf.org/html/rfc4648).
Further documentation and information can be found at http://www.haskell.org/haskellwiki/Library/Data_encoding.
Chop up a string in parts.
The length given is rounded down to the nearest multiple of 4.
- PEM requires lines that are 64 characters long.
- MIME requires lines that are at most 76 characters long.