Add support for Riak packages other than OSX

At the moment the Rakefile does this:

```
RIAK_DOWNLOAD_URL = "http://s3.amazonaws.com/downloads.basho.com/riak/#{RIAK_VERSION[0..2]}/#{RIAK_VERSION}/osx/10.6/riak-#{RIAK_VERSION}-osx-x86_64.tar.gz"
```

It assumes that the environment is OSX, with some basic platform checking it should be possible to download the right Riak package for most platforms.
