Allow Hiera merge lookup types

I believe [HI-118](https://tickets.puppetlabs.com/browse/HI-118) prevents merge lookup types from operating as expected when hiera_hash or hiera_array is called within the class parameters section.

This prevents the $users variable from being passed in as a parameter, but shouldn't we be modeling this data in Hiera anyway?
Merge pull request #4 from jbartko/feature/merge-lookup-types

Allow Hiera merge lookup types