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

[chearon]

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!

[mente]

You can create new branch and merge changes from upstream. Then apply retina related commits via git diff && git apply. This way you can separate commits instead of pushing all of them.

[mente]

Forgot to mention: drop own commits via git reset --hard before merging upstream. And only then apply them manually. Hope this helps to get this PR faster in upstream

[chearon]

Yeah, crap, everything before the merge is irrelevant. I just used git checkout -b retina-fixes butler/master to start a new branch that's at the same HEAD of the master here...does that achieve the same effect? Then I'll do the manual commits...guess I should start developing features on different branches

[richardbutler]

@chearon Apologies for the delay on this one. It would introduce a breaking change, so would push onto 0.5, but if you want to add changes to the readme and example, I'll happily look into it a bit more as you make a good case for your use case.