Put it as a wrapper only (invariant from underlying modules)

Proposed changes:
- Convert  it as a wrapper only (invariant from underlying modules), hence we can swap the underlying `bcrypt` modules (native, or pure js) accordingly
- Add setup dan example on `README`
- Add some tests using `co` and `mocha` as its runner

Quick look on the API
```js

// e.g. using nan-bcrypt
var mod = require ('nan-bcrypt');

// or using pure js
// var mod = require ('bcryptjs');

co(function * (){
  var bcrypt = wrap(nan);
  var salt = yield bcrypt.genSalt();
  var hash = yield bcrypt.hash('tobi', salt);
  var compared = yield bcrypt.compare('tobi', hash);
})(done);
```