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.
This makes it safer, although I'd also be curious to know what code calls `placeMarkers` at such a time that this expression is true (i.e. the selection is outside of the Scribe DOM).