Added named arguments support for JSON_RPC

Added code to handle named arguments. As Ruby doesn't provide native named arguments support, code only finds out correct order of input arguments and sends values in order method is expecting. As skipping missing optional arguments makes problem, it sets all skiped arguments to nil, so you should define optional arguments like this:
def sum a, b=nil, c
  b ||= 5
  a + b + c
end
It's a bit annoying, but this solves all problems and gives full named arguments support.