fixed: some variable is not defined

[joe223]

change:

1. Use Strict Mode.
2. It works well on Node.js & 微信小程序 enviroment.

eg:

// jsencrypt/lib/jsbn/base64.js
function b64tohex(s) {
  var ret = ""
  var i;
  var k = 0; // b64 state, 0-3
  var slop;
  var v; // new code: v is not defined
  for(i = 0; i < s.length; ++i) {
    if(s.charAt(i) == b64pad) break;
    v = b64map.indexOf(s.charAt(i));
    ...