Use the `target` field of events, insted of `srcElement`.

This fixes #24.
Kudos to @mitar for the tip.
Revert "Correctly scan on cross-page selection."

We should just call documentChanged() before scan to force it.

This reverts commit 2dcedb329f08004e4a025974bbda4cdf7e4ed032.
Merge branch 'master' into peerlibrary

Conflicts:
	src/dom_text_mapper.coffee
Force rescanning on cross-page selection.
Merge branch 'cross-page-scan2' into peerlibrary