[jsfm] make private method as functional as possible

[Jinjiang]

e.g.

Vm.prototype._xxx = function (a, b, c) {
   // this
}

function Vm() {
  this._xxx(1, 2, 3)
}

will be converted to:

function xxx (vm, a, b, c) {
  // vm
}

function Vm() {
  xxx(vm, 1, 2, 3)
}

benefits:

1. protect the private method not exposed to developer
2. easy to trace the method source through import
3. easy to test
4. possible to minify the name (prototype member names cannot be minified)