Add event support

Implements code review feedback from https://github.com/ooyala/go-dogstatsd/pull/2
The intent is to give clients that use Event full control, but I see what you mean about missing out on the default namespace-based SourceTypeName if it's left empty on the struct. My first inclination is to modify Event(...) use EventOpts.SourceTypeName if it's not empty and otherwise fallback to the Client.Namespace behavior. A possible problem is that this leaves no way for the client to omit SourceTypeName from the message entirely. Is that something we care about?