Added hiera hash merge support

Added support to use a hiera hash merge instead of the automatic priority lookup.

I found it useful for creating bridged vlan interfaces on our Xen servers via a hostgroup hiera file and assigning the IP for each server in its own hiera file.

The down side of this approach is that if there is **any** hiera data returned it will be used regardless of any parameters passed in a class{} declaration.  The only alternative to I could come up with that keeps the same ordering of parameter lookup was to use a separate hiera key.  I tested the separate hiera key but found it felt sort of unintuitive since a hiera priority lookup of the regular parameter name would override the merged hiera only key.  My testing on this alternative can be found in my [hiera-separate-key](https://github.com/szynaka/puppet-network/tree/hiera-specifc-key) branch if you're interested.