update README for v0.8.1

version bump to 0.8.0; update README, contributors etc.

PyPI release 0.7.5

update README

update README for 0.7.3

README: update benchmark table

strip build status badge from PyPI README

update README: mention old Python pre 2.6.5 support

update README for version 0.6 + note about tags in HOWTOPUBLISH

update README for version 0.5.1