Fix inheritance for IDBStore

I found this because extending `IDBStore` with CoffeeScript doesn't work. Normally I would have used `_.extends` instead of replacing the prototype, but since nothing else depends on underscore I didn't want to add it.

Even if you aren't interested in CoffeeScript you can see that replacing the prototype breaks normal Javascript behavior.

    var ConstructorA = function() {};
    ConstructorA.prototype.test = function() {
        console.log("testing");
    };

    var ConstructorB = function() {};
    ConstructorB.prototype = {
        test: function() {
            console.log('testing');
        }
    };

    // true: normal behavor
    console.log(ConstructorA.prototype.constructor === ConstructorA);

    // false: IDBStore's current behavior
    console.log(ConstructorB.prototype.constructor === ConstructorB);