eliminate seek()s

Fixes https://github.com/ged-lab/khmer/issues/633
Zipfile support was added in May but has yet to appear in a released version of screed. I find that zipped sequence files are pretty rare. I just ran a test and the original code doesn't work (you can't call `readline()` on a ZipFile object). 

This behavior (or the support for Zip files at all) is not documented. Testing with larger archives revealed that Python 2.6 cannot stream zip files after all.