Clean up lua source a bit, added a type check for first post param.
Added LogMessage wrapper.

It has a simple Message.new() constructor, and
it's __newindex and __index metamethods are overridden.
So, for example, you can create a normal log message this way:
log_msg = Message.new()
log_msg.MSG = "Hello World!"

LuaSource "post" function now accepts a LogMessage as a second parameter.