Event object array

Currently `L.Evented.on` accepts an object containing events as follows:

```
EventedObj.on({
  eventName: FunctionA
});
```

The attached commits extend `.on` to accept an array of methods, for example:

```
EventedObj.on({
  eventName: [FunctionA, FunctionB, FunctionC]
});
```

I find this pattern useful, would appreciate peer review.

Adam