Added Andi Kleen's Snappy compression C implementation

http://github.com/andikleen/snappy-c
Added snappy and gzip compression support to the producer
Attempt to fill the batch queue before sending messagesets

This increases throughput under heavy load.

Controlled through properties:
 queue.buffering.max.ms
 batch.num.messages
Merged Makefile
Updated compression doc in rdkafka.h
Create INTRODUCTION.md - explaining the concepts of librdkafka
Make librdkafka build on MacOSX (Issue #19)
Added link to INTRODUCTION.md
Corrected performance number
Solved Makefile conflict