fix issue with responding 304 (Not modified)

in https://github.com/graphite-ng/carbon-relay-ng/issues/30 there is issue with serving old data even if it was changed
it seems that not giving reasonable ModTime makes http.FileServer to do some optimisation.
Merge pull request #2 from pwielgolaski/patch-1

fix issue with responding 304 (Not modified)

Currently the creation time of each artifact is set to Unix epoch time (0). Hence when changing the
artifact, subsequent calls would be cached by the browser.

TODO: Use etags.