xml-rpc client lib?

[naveedhd]

I was searching for the same a while ago but couldn't find any. But 
implementing on top of `std/xmltree, xmlparser` with `net`/`httpclient` seemed 
doable with my limited experience with both Nim and XMLRPC. Here is some 
scribbles: <https://github.com/naveedhd/rosnim> (in the context of ROS client 
library which used xmlrpc)