Added base64 encode/decode support

This branch enables base64 coding for some url or API use.

Proposal: https://docs.google.com/document/d/1QKVo-ylGLhCHqV5JcAnA4QwMfw30T7-ZS4EuC6CU-3c/edit?usp=sharing

Try the GAE app on http://trim-mode-485.appspot.com/
Sample app: https://www.google.com/url?q=https%3A%2F%2Fdrive.google.com%2Ffile%2Fd%2F0B6pgfXvNtaFyUkZfT1RTMFBILUk%2Fedit%3Fusp%3Dsharing