Add output panel as an option to show argument hints

Refer to Issue #47.

Add a preference setting: "tern_argument_hints_type", which can be
set to either "status" or "panel". If "tern_argument_hints" is set
to True, this setting determines how the arghints are shown.
"status" is the default value and uses the old status bar mechanism,
while "panel" uses an output panel to show the arghints.