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
```
The complex description. For me it's hard to understand.
What's about the following?
```
Disable using batch. You also can specified an array with method names which shouldn't be batched.
```

And add an example for both cases to Readme.md