Fix HiDPI CSS dimensions, make the selector option more powerful (fixes #17)

The 2 most recent commits simply fix HiDPI so that on retina devices, the images arent twice the size they're supposed to be.

The 3rd commit was discussed on #9 (you didn't seem convinced, but hear me out).  I wish I'd developed them separate branches, because the retina patches are what really need to be upstream.  But essentially, this makes the `selector` option more intelligent.

Example: `selector` option is set to `.spriticon` and spritesheet calls `resolveImageSelector` on a file which returns `.icon-outer .file`.  It only makes sense to generate the selector `.icon-outer .spriticon.file`.

It essentially does the same thing it used to except that it knows to apply the rule to the last selector in a space-separated hierarchy.  It also expects `resolveImageSelector` to return a real selector, rather than assuming it to just be a single class name.

Anyway, take it or leave it.  I found it useful and sensible.  Thanks!