Tarball iterator

Nick/Aaron,

This is a small modification to poretools to allow a bit faster processing, you can stream out of the tarball now. So for example `poretools readstats Ecoli_R7_ONI.tgz` will start outputting numbers immediately.

A small (?) trade-off to achieve this -- `get_num_files()` is now lazy for tarballs, so it will take a long time the first time it's called. I found only one usage of `get_num_files()`, in squiggle plots, and rewrote that utility so it doesn't use this method, and it works a lot faster now.

--Misha