Add syntax highlighting to code examples

This also includes some minor tweaks and adjustments:

* Adding links to referenced files in the repository
* Adding backticks around references to project files
Merge pull request #22 from gevans/readme-tweaks

Add syntax highlighting to code examples
Fail gracefully when Rack params cannot be parsed /cc #23
Inform user of method when rack_request.method fails. (#23)
Release 1.6.2
Merge git://github.com/honeybadger-io/honeybadger-ruby into assync_notify
typo. update to readme and changelog