Allow pass several keys in one udp packet

This patch allows write
```Lua
statsd:gauge{
  processor = processorCounter;
  memory    = memoryCounter;
  disk      = diskCounter;
}
statsd:increment{"foo", bar = 2} -- foo=foo+1; boo=boo+2
statsd:decrement{"foo", bar = 2} -- foo=foo-1; boo=boo-2
```
For now I do not check UDP packet size.

Also I think about common functions that allows combine different types of stats in one packet
```Lua
statsd:stat{gauge = {...};counter={...}}
```