Added payload arg name config and scope merge config

Added ability to specify the argument name for the HTTP request body
value that is passed to your service method. (Still defaults to
"Payload".)
Also, added ability to control which scopes are merged into the
arguments scope that is passed to your service method. (Still defaults
to all of them.)
Moved DefaultArguments config into Arguments.Defaults

Now that there is an "Arguments" config, it made sense to move the
"default arguments" config into that place in the config object.
Merge pull request #20 from cfchris/parameter-controls

Parameter controls