check for existed resource and skip downloading

[terrancewong]

Not downloading everything again is much appreciated.

[MrChromebox]

we're talking <1mb of additional data downloaded. I could checksum everything and verify before re-downloading but that just seems like an extra hassle given the small benefit.

[terrancewong]

just slightly more friendly to slow connections and multiple offline flash :-P

[MrChromebox]

if you've modified my script for offline use, then adding a check for the required utilities and not downloading them should be trivial ;-)