Add "lower" & "upper" matcher functions

To facilitate case-insensitive string matching for instances like MAC addresses, functions "lower" and "upper" have been added to the matching language. This will allow, e.g. `["=", ["upper", ["fact", "macaddress"]], "F0:1F:AF:D8:F2:A6"]`

Fixes https://tickets.puppetlabs.com/browse/RAZOR-89