Configuration to run coveralls on py33
Separate tox env for coverage check
Allow TOXENV=cov to fail in travis-ci
Install coveralls only when needed
Fix allow_failures syntax

allow_failures.env requires a string when matrix/global is not used?
Also, indent is changed to fit with other entries.
Merge pull request #198 from tkf/coveralls

Run coveralls task separately on travis-ci (fixes #196, refs #197)