Move the content_type check outside of `data!`.

Receiving an actual invalid content_type would try to pass the Rack::Response to
the `PayloadParser` and throw an exception.
Update README formatting
Merge pull request #134 from tombell/readme-fixes

Some README formatting fixes
Merge pull request #133 from tombell/content-type-check

Update Content-Type check in `Janky::GitHub::Receiver`
ci status should also be listed via ci help

All 3 different formats supported
ci status - lists all jobs
ci status janky - lists status of janky
ci status janky/master - lists status of janky/master branch