Add testing framework and tests

Hey there,

first of all - thanks for the puppet hiera module!

I'd love to apply some changes, and refactored those to make it mergable back into your repository.

- incorporate test skeleton from https://github.com/Jimdo/puppet-skeleton. this adds plenty of testing possibilities. at the moment i just implemented a basic set of unit tests, but it adds stuff like integration testing with vagrant boxes.

- enhance the "extra_config" logic, by enabling handling for hash values. this way more complex extra configurations can be given to the module. backwards compatibility for plain strings is ensured and tested.

Last point: would you mind changing the name to "puppet-hiera"? This seems to be defacto standard for puppet modules, and does not break automatisms on our side.

Thanksalot!