Refactor base64 string functions out of helpers.js and into stringview.js

This PR moves the base64 string handling functions that were originally copied from [mozilla docs] (https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding) into [stringview.js](https://developer.mozilla.org/en-US/Add-ons/Code_snippets/StringView).  They are the exact same functions but wrapped in a library that is maintained by mozilla.

This helps address https://github.com/WhisperSystems/TextSecure-Browser/issues/16.