Thread safety and debugging options

[jlopez]

• thread_safe constructor argument enables _stats
  single-threaded access to support use of single instance by
  multiple threads
• debug constructor argument enables stdout printing of every
  metric sent to statsd
• host may be None in which case network access is turned off

[jsocol]

@jlopez I've gone a bit of a different way here by adding a pipeline concept. It looks something like:

pipe = StatsClient().pipeline()
pipe.incr('foo')
pipe.send()

The StatsClient itself should still be threadsafe, but Pipeline objects are not. This is similar to how redis-py works, and I think it simplifies things and makes sense.

The debugging options would be nice, but it might make more sense to implement a DebugClient as a subclass, overriding _send to just print, rather than adding a kwarg. Either option requires changes at the client init site. Any thoughts?

[jsocol]

Closing this in favor of the pipeline method. See #20.