lua-dest: global configuration now stored in a "hidden" variable in lua state.

It will be needed for some Lua-wrapped object, and the only place,
where it can be stored is in the Lua state.
lua-dest: Message class name moved to header.

It will be needed in template wrapper.
lua-dest: Added Template class.

Template class is a wrapper over syslog-ng LogTemplate struct.
It allows templating in raw mode, and it allows to create multiple
template instances per lua destination.

Example usage (raw mode):

function init()
  template = Template.new("${MESSAGE}")
end

function queue(msg)
  print(template:format(msg))
end
Merge pull request #15 from talien/feature/lua-template

lua: add a LogTemplate wrapper class to be used in raw mode