allow digits in config vars
Correct description for overwrite example
Add option to specify keys
Merge pull request #4 from nicholasjhenry/master

Small correction to README
Revert "Merge pull request #4 from nicholasjhenry/master"

This reverts commit 5e5dac6f744e39f1e42884efab4727ea89ba9c02, reversing
changes made to 561d5925ee7ae4cd7e08eb0d617180ab5f424c38.
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
Merge pull request #6 from nicholasjhenry/upgrade-to-api

Upgrade to use the Heroku API
Correct description for overwrite example
Merge pull request #7 from nicholasjhenry/readme-fix

Correct description for overwrite example
merged in upstream