Document how to make local coverage reports

The coverage package is implied by our travis script but not explicitly stated
in the readme.
Add coverage badge to the top badges.
Merge pull request #34 from Moredread/moredread/feature/coverage_top_badge

Add coverage badge to the top badges.
Merge pull request #33 from aepsil0n/fix/document_local_coverage

Document how to make local coverage reports
Merge branch 'master' into docs/contact_mail

Conflicts:
	README.rst