Minor changes for convenience

This pull request cover two minor changes to make the library slightly easier to use:
* I have added methods to `statsd.client.Client` which give back an instance of whichever object type you have requested. I think this gives a cleaner interface than using the existing `get_client()` way.
* Made `statsd.timer.Timer.start()` return the timer instance so you can chain the instantiation and start of the timer, while still assigning the timer to a variable like this:  
`timer = Timer('application_name').start()`