Delete method shouldn't reset model's id

Model's ID should not be deleted (reseted) in the adapter. The problem is that after the `Sync` is done Backbone will trigger `destroy` which will execute a method called `_onModelEvent` -> `remove` ,but model doesn't have an id anymore ,so it won't be deleted from a collection - set `_byId`. If you try to add a new model to the collection with the same ID, backbone will not allow you to do that because the ID already exists in the set.

Example:

test.js (model)
~~~~javascript
exports.definition = {
	config: {
		columns: {
		    "id": "INTEGER PRIMARY KEY",
		    "whatever": "TEXT"
		},
		adapter: {
			idAttribute: "id",
			type: "sql",
			collection_name: "test"
		}
	}
};
~~~~

index.js (controller)
~~~javascript
var testCollection = Alloy.createCollection("test");

var model = Alloy.createModel("test", {
	"whatever": "..."
});

model.save();

testCollection.add(model);

console.log("Models in `testCollection`: " + testCollection.length + ", Model ID: " + model.id);

model.destroy();

console.log("Models in `testCollection`: " + testCollection.length + ", Model ID: " + model.id);


var newModel = Alloy.createModel("test", {
	"whatever": "---"
});

newModel.save();
testCollection.add(newModel); // Won't add this model

console.log("Models in `testCollection`: " + testCollection.length + ", Model ID: " + newModel.id);
~~~