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
```
OK, I'm convinced that the options shouldn't be extended in the constructor.
But I also think that we shouldn't check the options each time the `Api.exec` method is called. My solution is to create a private property `_noBatchingForMethods` in `Api` which will contain an array with the names of the methods that shouldn't be batched. And it will be created in the constructor.