Rethink Backbone.sync

As we all know, there are numerous plugins in the wild which override `Backbone.sync` with specific domain logic. Things like storage proxies, web socket connections, etc. The way plugins add this functionality tends to be messy and conflict with one another.

For Backbone v2 I suggest we reimplement how a custom sync should be added to Backbone. I've long suggested these plugins export a `CustomCollection` and `CustomModel` with a custom `sync` function implemented there, as opposed to globally overriding BB.sync; however they both have some merit. I would suggest something like the following adding a custom sync

``` js
function predicate(model, options) {
return model.useProxy;
}

function syncFn(model, options) {
// sync function as normal
}

// Whether this sync method should stop other syncs from being checked.
// This could be useful for storage syncs which wish to add data to an indexedDb
// while also executing an http sync
var exclusiveSync = false;

Backbone.addSyncMethod(predicate, syncfn, exclusiveSync);
```

Start by reading the current Backbone.sync entry point and reviewing how plugins override it. Compare the proposed predicate, sync function, and exclusiveSync behavior with the existing extension approach. Done means a settled custom-sync design with clear interaction and compatibility expectations, but the issue does not identify specific files or tests.

Written by the indexing model from the issue text.