editor.doesElementContainSelection

Includes doc and tests.
Use `setCaretInSelector` in doesElementContainSelection
doesElementContainSelection: optimization.
doesElementContainsSelection: fix for IE7&8
doesElementContainSelection: slightly better doc
Changelog entry for `doesElementContainSelection`.
Merge branch 'master' into doesElementContainSelection_master
doesElementContainSelection: Fix extraneous code execution.
doesElementContainSelection test: non-collapsed twice nested
Merge pull request #637 from wymeditor/doesElementContainSelection_master

`editor.doesElementContainSelection`