Adding Rake-based toolchain to bootstrap, build, and control NGINX

[davidalber]

This pull request adds a Rake-based toolchain that was adapted from the system in this tutorial. This adds the following tools:

• A bootstrapping system that downloads and installs NGINX and librdkafka.
• A command to compile NGINX with the module code.
• Commands to start and stop NGINX.

So now, assuming the dependencies of librdkafka are available on the system, getting the module up and running looks like this:

$ bundle install # Requires bundler, described in README
$ rake bootstrap
$ rake nginx:compile
$ rake nginx:start
$ # plus starting Kafka and creating the topic in the nginx.conf file

Additionally:

• This adds an nginx.conf file that is symlinked into the bootstrapped NGINX.
• Updates the README file with instructions on how to use the toolchain.

The nginx.conf file I checked in only has one location endpoint, whereas the previous example in the README had two location endpoints with separate topics. I went with one because I thought it lowered the barrier to start and test for a brand new user, but having two endpoints had the advantage of showing clearly that separate endpoints can write to different topics. If you like the changes in this PR but want a bigger example in the nginx.conf, let me know.

Like I said in PR #4, don't hesitate to decline this if you don't like the direction it's going. I just wanted to contribute back since your code helped me.