Add asynchronous requests to client

[2opremio]

As a user who is instrumenting a service with fluent logging, I am concerned about blocking requests affecting my service in different scenarios:

1. The fluentd server going down or degrading.
2. Fluent requests slowing down the requests being processed by my service. I'd rather stop logging than degrading the performance of my service.

As a result I ended up incorporating asynchronous logging through a worker goroutine reading from a small memory buffer on top of your Go client. If the buffer gets full for whatever reason (logging slowing down, fluent going down ...), events will get dropped but the service won't be affected.

Admittedly, if my service goes down, the events inside the buffer will be lost, but I am perfectly fine with that.

I believe others would benefit from this scheme and probably should be shipped by default with the client.

My code is pretty ad-hoc at this point but I may consider cleaning it up and upstreaming it at some point.