Add support for counters

Add basic support for counters in Riak 1.4 and higher.

Usage:

```go
// Get counter from bucket, loads value
c = bucket.GetCounter("key")

// Get counter without existing bucket instance, loads value
c = cli.GetCounterFrom("bucket", "key")

c.Value                  // 0
c.Reload
c.Increment(1)           // 1 on server, 0 in strucct
c.IncrementAndReload(1)  // 2 on server and in struct
c.Decrement(1)           // 1 on server, 2 in struct
c.DecrementAndReload(1)  // 0 on server and in struct
```

The "AndReload" methods exist to take advantage of an option in update that returns the current value, thus saving a req/resp cycle.

The format of message_codes.go was updated accomodate new codes that aren't continuously numbered. 

Contributes to #40