Added argument in people.set people.set_once & tracker.alias to accept a buffered_consumer

This will allow batch jobs of sending many requests to Mixpanel much faster (~30 times faster).
Please see comments in people.rb line 48 for an example on how to use. 
by default @sink = consumer.method(:send) in initialize(token, &block). If a block is passed in it will set @sink = block.