<shake> command is added, based on move 

I need this to implement renpy's "with hpunch" / "with vpunch". Default
values of period and duration are taken from renpy sources. At the very
begining of shaking image is moved by dx, but it's exactly renpy's
behaviour, so i decided not to change it to smooth starting.

You can test it with something like this:

&lt;shake asset="backgrounds" dx="-15px" />
&lt;shake asset="backgrounds" dy="5%" period="80"/>
&lt;wait />

Also note "element.style.left = ox + xUnit;" at onFinish.
Maybe we need the same things at &lt;move> command.
Can you add default values for dx and dy? Would make the command easier to use.