Adding btoa polyfill

lib/helpers/btoa.js

+'use strict';
+
+// btoa polyfill for IE<10 courtesy https://github.com/davidchambers/Base64.js
+
+var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
+
+function InvalidCharacterError(message) {
+  this.message = message;
+}
+InvalidCharacterError.prototype = new Error;
+InvalidCharacterError.prototype.name = 'InvalidCharacterError';
+
+function btoa (input) {
+  var str = String(input);
+  for (
+    // initialize result and counter
+    var block, charCode, idx = 0, map = chars, output = '';
+    // if the next str index does not exist:
+    //   change the mapping table to "="
+    //   check if d has no fractional digits
+    str.charAt(idx | 0) || (map = '=', idx % 1);
+    // "8 - idx % 1 * 8" generates the sequence 2, 4, 6, 8
+    output += map.charAt(63 & block >> 8 - idx % 1 * 8)
+  ) {
+    charCode = str.charCodeAt(idx += 3/4);
+    if (charCode > 0xFF) {
+      throw new InvalidCharacterError('\'btoa\' failed: The string to be encoded contains characters outside of the Latin1 range.');
+    }
+    block = block << 8 | charCode;
+  }
+  return output;
+};
+
+module.exports = btoa