Per method batch disabling

This is only a preview. Tests and more docs are forthcoming. I just wanted to get the other maintainers opinion on this interface.

The per-method batch disabling will be configured in the frontend and it's done through the Api class constructor like so:
```
var api = new Api('/api', {noBatchingForMethods: ['slow-poke']});

api.exec('ordinary-method', {boringParam: 'boring value'}); // this method will be batched

api.exec('slow-poke', {huh: 'uh?!'}); // this method won't be batched
```
I agree that `this._options` should represent initial options, defined by the client, that is `this._options = options || {}`.
The code structure of the `api.exec` is up to the author, though, the version suggested above looks good, imho :)