Decodes string using MIME base64 algorithm

@author Tyler Akins (http://rumkin.com)
@param {String} data
@return {String}
function(data) {
			var o1, o2, o3, h1, h2, h3, h4, bits, i = 0, ac = 0, tmpArr = [];
			var b64 = chars, il = data.length;

			if (!data) {
				return data;
			}

			data += '';

			do { // unpack four hexets into three octets using index points in b64
				h1 = b64.indexOf(data.charAt(i++));
				h2 = b64.indexOf(data.charAt(i++));
				h3 = b64.indexOf(data.charAt(i++));
				h4 = b64.indexOf(data.charAt(i++));

				bits = h1 << 18 | h2 << 12 | h3 << 6 | h4;

				o1 = bits >> 16 & 0xff;
				o2 = bits >> 8 & 0xff;
				o3 = bits & 0xff;

				if (h3 == 64) {
					tmpArr[ac++] = String.fromCharCode(o1);
				} else if (h4 == 64) {
					tmpArr[ac++] = String.fromCharCode(o1, o2);
				} else {
					tmpArr[ac++] = String.fromCharCode(o1, o2, o3);
				}
			} while (i < il);

			return tmpArr.join('');
		}