Fix visible observed attribute overwriting DOM

This fixes an issue where using the `visible: true` visibility option in a binding
will cause the value of the observed attribute to be set via the `$.text()`
jQuery method on the selected element.  An example of the behavior can be seen
here: http://jsbin.com/udapeb/8/edit

This patch sets `updateView` to `false` for `visible: true` bindings.
Additionally, there is an existence check in the default text binding handler.

Tests updated to verify the content of the bound element.