enable multiple hiera backends to be specified, including configuration parameters
Merge remote-tracking branch 'robin/develop'

Conflicts:
	README.md
	manifests/init.pp
	templates/hiera.yaml.erb
adapt tests to changes from robinbowes

as additional configuration is now handled via "backends" beeing hashes
with key=>value options rather than strings, extra_config and it's tests
(string/hash) are no longer needed.
Merge pull request #1 from Jimdo/incorporate_changes_from_robinbowes

Incorporate changes from robinbowes