Merge pull request #469 from starwed/master

Dirty rectangles should track MBR
Add guard before accessing target.nodeName

We need to check that target exists before accessing its properties!
Merge pull request #471 from starwed/control-addGuard

Add guard before accessing target.nodeName
Merge pull request #470 from starwed/master

Use floor/ceiling instead of round when calculating MBR.
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
avoid unnecessary function call in _attr

this.pos() is calculated even when it's not used. Actually, it's never
used (because mbr() already has code to call this.pos() if necessary.)
So this is a wasteful function call and object creation.

Also, fix typos.
performance: Only call _cascade() when there are children

This should help performance: With many childless moving sprites,
this can avoid hundreds of unnecessary function calls per frame.

(Also fixed documentation of _cascade(). It's a bit vague, but that's
better than being untrue!)
bump version, update copyright year
Merge pull request #481 from sbyrnes321/update_copyright

bump version, update copyright year
Merge pull request #479 from sbyrnes321/optimize_attr

avoid unnecessary function call in _attr
Merge pull request #480 from sbyrnes321/_cascade-improvement

performance: Only call _cascade() when there are children