Add ability to specify function as 'observe'
added changes for observe function handling
Clarification in docs for #84
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 updateView to false for 'visible' bindings.
Additionally, there is an existence check in the default text binding handler.

Tests updated to verify the content of the bound element.
Update to allow for opt-in via `updateView: true`

* `updateView` now defaults to `false` with `visible: true`
* `updateView: true` opts in to view element value binding
* Tests updated for checking visibility in the normal case, and value in the
`updateView: true` case
* Documentation updated to reflect opt-in nature