Elm base64 encoding

[marceloomens]

Implement native Elm support for base64 encoding. I currently rely on base64-js integrated with a fork of simonh1000/file-reader.

See also:

• Add base64 support simonh1000/file-reader#27
• https://groups.google.com/forum/#!msg/elm-discuss/u_ui3PlDwLc/uijKIsegBQAJ;context-place=forum/elm-discuss