Size specifications, IE7 fix and dynamic styles

[eric1234]

When using this polyfill I found the following issues:

• It was caching the size and position info. This isn't compatible   with this information getting changed dynamically (for example   through @media styles).
• Added support for specifying exact sizes.   For example background-size: 100% 100%
• Fixed issue where top was not working because 0 evaluates to false.
• There was a extra comma that caused problems in IE7.

This is a rough patch. I basically just fixed it up for my website, copy and pasted the final code into GitHub and submitted the patch in hopes it is useful to someone else. If I get a chance down the road I can try to make a proper patch (commit for each issue I am fixing and more extensive testing to make sure I didn't break anything). But I figured a messy patch is better than no patch if this helps someone out and I never get a chance to clean it up.

[jefferyto]

Thanks for this patch! I wasn't able to use you code as-is while developing 0.2.0 but your ideas are definitely in there :-)