Support argument conversion with types not supported by XML-RPC

[pekkaklarck]

We have #83 about adding get_keyword_types that will add basic argument conversion support. Basically RemoteServer will return type information to Robot and it will do argument conversion based on it. It won't be enough with types that are not supported by XML-RPC, though, because the Remote library needs to convert them to strings. We need to enhance RemoteServer so that it converts these strings back to correct objects. Because we know the exact format returned by Remote, this conversion isn't too hard. There's some work setting up the overall conversion infrastructure, though.

Related to this, we need to look is conversion to strings always the best approach with types not supported by XML-RPC. For example, with something like timedelta, returning a float got from timedelta.total_seconds() could be a better idea. I'll submit a separate issue about that to Robot's tracker.