Make scss build atomic

Since using scss, whenever I refresh in the browser during development I
often get unstyled content. My pattern has been to run `make css` under
watch, and I guess this clobbers the target gittip.css in a non-atomic
way. This is a small change to the Makefile to build gittip.css in a
temporary location and then atomically move it into place to be served.
This doesn't make it happen any faster but at least prevents unstyled
refreshes.