readme says plugin works with jQuery 1.4.2+ but .prop() only works on 1.6+

[millermedeiros]

$(event.target).prop('contenteditable') == 'true' will fail on jQuery prior to 1.6, it should be written as $(event.target).attr('contenteditable') == 'true' in older versions.

I added a note in the source code about it: https://github.com/tzuryby/jquery.hotkeys/blob/321e02b98be973d1ea09eb97ceb921411ffb3415/jquery.hotkeys.js#L51

Even thought about adding some conditional check for the version but decided not to.

Anyway, just documenting it here in case anyone is having this problem (main reason why I updated the test files to use jQuery 1.7.1)

cheers.