Fixes #204 and #153.

Makes sure ```this.range``` is defined when calling ```selection.getContaining``` and ```selection.placeMarkers```. Makes sure the current selection is inside the scribe element before placing markers. Fixes #204 and #153.
Here is a reproduction. Just click the Set Content button 10 times and right click + inspect.
http://jsfiddle.net/npqf0Lgu/1/

Since in our use case we need to programmatically update the contents of the scribe, this is what happens (em markers are left around all over the document).

Additional explanation is at end of #153