replace Config.heroku_api_key for api_url, easier to change/stub
adding a little heroku api stub
add expects to excon call
add collab api to the stub too
move headers to base Excon instance so we can reuse on other calls
filling in with HerokuClient.account_info
cosmetics
update readme for new config var format
support multiple/arbitrary API headers
reuse api connection
use JSON instead of newlines
Merge pull request #5 from heroku/wl-pedro-amend

use JSON instead of newlines