some improvements to text

Where possible, avoid styling text using css, as it makes it harder to
switch DOM <--> canvas (see complaint at
http://buildnewgames.com/game-engine-comparison/), and doesn't always
work anyway. Also, add .unselectable() method
clarify wording in docs ("text elements")
Merge pull request #474 from sbyrnes321/textcss

some improvements to text and documentation thereof