A Behavior's $el incorrectly calls the view's $el as a function

1. The current way `Behavior#$el` works is incorrect. It attempts to call `this.view.$el` as a function. However, it is a jQuery **object**, not a function. Therefore, you'd get an error when attempting to call `this.$el()` from inside of a Behavior. I've fixed `$el` to always return the `view.$el` as an object.
2. The documentation states that `$el` is a cached jQuery selector. However, it is defined as a function. I think that It *should* be defined as function because the `$el` could change on the view. I've corrected the documentation for this.