Add Mixins and Support Browsers ie7+

I went ahead and added mixins to support ie7+.

Also, you guys aren't using prefixes for ``-moz`` or ``-o``, so that's missing out on a fair amount of browser share.  I fixed all that.

Finally, you aren't using nested selectors, which is an awesome feature in LESS.  I fixed (some) of that too.

Take what you want from these commits.  I'm going to keep at it on my repo, as I make themes using warp.