Cache build outputs

Cache build outputs into the TMP directory. This should be more useful than just the modified response headers = which can be ignored by browser hard refreshes.

Refs #218
Also on a performance note, I'm not sure what the cost of `chmod` is, or touching the filesystem in general, but that line is pretty much executed on each page load, whether or not the build files will be regenerated. Maybe it could be moved below, when writing actually occurs?