Feature/add update

This PR adds two new features:

1. like/nlike support for memory connector
2. Add a new API for update(where, data, cb)

/to @ritch 
/cc @bajtos 
nitpick:

I prefer

```js
switch(arguments.length) {
  case 1:
  break;
  case 2:
  break;
  default:
  break;
}
```
over `else if ... else if ...`