Don't call callback if not provided when using mock instances

When creating a mock instance, if a callback isn't provided, the code fails when it tries to call the callback param:

```
TypeError: undefined is not a function
    at Client.send (/Users/dave/Sites/repos/butter/node_modules/node-statsd/lib/statsd.js:178:5)
    at Client.sendAll (/Users/dave/Sites/repos/butter/node_modules/node-statsd/lib/statsd.js:148:10)
    at Client.increment (/Users/dave/Sites/repos/butter/node_modules/node-statsd/lib/statsd.js:71:8)
```
This fixes patch fixes that case.