Add build badge

TODO: Remember to change hugovk to sixohsix after merge
Merge remote-tracking branch 'upstream/master'

Conflicts:
	README
Make tests more robust when run in parallel, to avoid the occasional https://travis-ci.org/sixohsix/twitter/jobs/23153488
Fix test typo
Merge (parallel tests had already been fixed in upstream)
Remove redundant code
Add coverage to tests
Ignore pypy lib files like /usr/local/pypy/lib_pypy/datetime.py
Add Coveralls coverage badge [CI skip]
Coverage badge points to sixohsix [CI skip]

It'll need enabling on coveralls.io and a build to run to show a number
Merge pull request #226 from hugovk/master

Add coverage to tests