Tidy up README

Merge pull request #613 from PythonCharmers/lwan/0.18.3-release

Merge pull request #523 from jmadler/master

Prepare for 0.18.2 release

Merge pull request #513 from PythonCharmers/v0.18.x

Merge 0.18.1 into master

Merge pull request #505 from jmadler/master

Prepare 0.18.0 release

Merge pull request #406 from jmadler/v0.17.x

V0.17.x

Fix py2.6 compat issues

Update What's New doc to mention that issue #141 is fixed in v0.16.0

Merge branch 'v0.15.x'

Merge branch 'v0.15.x' and tag 'v0.15.1'