allows upstream to override credentials. fixes #18
remove superenv from core lib, bins use it and set credentials via API
Merge branch 'master' of github.com:mapbox/cfn-config into cred-override

Conflicts:
	bin/config.js
	bin/create.js
	bin/delete.js
	bin/info.js
	bin/update.js
Document setCredentials()
Merge pull request #22 from mapbox/cred-override

Allow upstream to override credentials