Correct description for overwrite example
Add feature to specify config keys
Upgrade to use the Heroku API

* This eliminates the various 'DEPRECATED' notices caused by using the Heroku::Client with recent versions of the Heroku gem
* Fixes issue #3
Update error message
Clarify heroku-api requirement
Correct description for overwrite example
Merge branch 'readme-fix'
Merge branch 'upgrade-to-api'
Merge branch 'specify-keys'