Add getparam function to get defined resource parameters

As far as i know there's no other puppet-dsl-like way to get parameter of
defined resource, so that's why i implemented getparam function, which takes
resource reference and parameter name and returns parameter value.

    Here's another example why this function is really useful:

    define config($path, $config_param1, $config_param2) { }

    define example_resource($config) {
        $path = getparam($config, "path")

        notice("Path is $path")
    }

    define example_resource2($example_resource, $config = getparam($example_resource, "config")) {
        $config_param1 = getparam($config, "config_param1")

        notice("Config parameter is $config_param1")
    }

    define example_resource3($example_resource, $config = getparam($example_resource, "config")) {
        $config_param2 = getparam($config, "config_param2")

        notice("Config parameter is $config_param2")
    }

    class test_getparam {

        config { "config_instance":
            path => "/some/config/path",
            config_param1 => "someconfigtext1",
            config_param2 => "someconfigtext2",
        }

        example_resource { "example_resource_instance":
            config => Config["config_instance"]
        }

        example_resource2 { "example_resource_instance":
            example_resource => Example_resource["example_resource_instance"]
        }

        example_resource3 { "example_resource_instance":
            example_resource => Example_resource2["example_resource_instance"]
        }
    }

    class { "test_getparam": }