Avoid errors with undefined populator, selector

For reasons I could not determine, and which disappear whenever you add a
debugger to the mix (that is, it is a race condition), The populator and helper
members are sometimes undefined causing errors. In addition, this code makes
sure we don't get the entire hierarchy if more than one parent matches
the selector, for we most certainly don't want more than one popup.

Fixes #408.
Sync with coffee
Rename mimeType to type inside keyDelay function

It overwrites the mimeType variable in the outside scope. Not harmful
as used here (yet) but likely not what was intended.
Add Plain Text option to math popover

This allows math to be converted back to plain text. Fixes #413.
Merge pull request #3 from oerpub/oerpub-ticket408

Avoid errors with undefined populator, selector
Merge pull request #5 from oerpub/oerpub-ticket413-pull

Add feature to math popover that allows user to set content back to plain text
Merge branch 'master' into oerpub-ticket414-pull

Conflicts:
	src/plugins/oer/math/lib/math-plugin.coffee
	src/plugins/oer/math/lib/math-plugin.js