Update README.md with new URLs

Varnishkafka has a simpler github project name now. Also, the separate
librdkafka repository for Debian doesn't exist and it was merged with
this one, in a separate branch.
Avoid typedef redefinitions (issue #18)
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
Create INTRODUCTION.md - explaining the concepts of librdkafka
Make librdkafka build on MacOSX (Issue #19)
Added link to INTRODUCTION.md
Corrected performance number
assert() is not marked as noreturn on some MacOS X versions
Dont use librd thread primitive
Fixed rd_kafka_destroy() resource cleanup (issue #20)
Added regression test harness
Added regression test for issue #20
Make sure messages in fligth are sent to other brokers in case of leader failure.

Move messages that are currently in flight (to the current toppar leader)
back on to the toppar message queue (prepend) so they are properly sent
to the new broker if a new one is elected.
Cosmetics
Proper -lrt linker flag on Linux
Removed outdated FIXMEs
ci-travis: build with clang also, and build examples.
Added more known users of librdkafka