Add the ability to pre-connect to the UDPSocket with `connect`

In testing with a previous version of the statsd-ruby gem we noticed
that a lot of time was spent sending these UDP messages. Looking
at the Ruby API, I noticed that by calling `connect` on the socket
ahead of time, future sends on the socket were significantly faster.

This adds a `connect` method, that when called, no longer needs to send
host/port with every send.

In testing in production, it has reduced calls by greater than two orders of magnitude

```
       user     system      total        real
udp with connect  0.010000   0.000000   0.010000 (  0.074522)
udp without connect  0.120000   0.530000   0.650000 ( 13.096515)
statsd with connect  0.000000   0.090000   0.090000 (  0.103520)
statsd without connect  0.100000   0.620000   0.720000 ( 13.483539)
```