Fix visible observed attribute overwriting DOM

This fixes an issue where using the 'visible' 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 the `updateMethod` to `null` for 'visible' bindings and adds an
existence check in the default text binding handler.

Tests updated to verify the content of the bound element.