Added enabled flag that'll allow to enable/disable fetch-cache globally

The main purpose of this change is to allow disable fetch-cache globally disregard caching set.
Mainly it's for tests, where you'd like to check that cached flag has been set to true but does not make actual caching and cleaning.

I guess regular enable disable should be done with
```javascript
App.caching_enabled = true;
collection.fetch({cache: App.caching_enabled});
```
This change is mainly for ease of testing.