Improve built-in support for formatted output

[pvdb]

By using the new "complex formatting" feature in Ruby 1.9, we can significantly improve the built-in support for formatted output.

By implementing the to_hash method on each log line, we could replace something like this:

puts [remote_addr, http_user_agent].join('  =>  ')

... with something like this:

puts "%{remote_addr} => %{http_user_agent}" % self.to_hash

... or even override the puts method on log line instances, and make the hash implicit in the DSL:

puts "%{remote_addr} => %{http_user_agent}"

/cc @jspc