Support for building documentation on Linux systems.

I noticed that docs use a hard-coded path for the icons directory for asciidoc that makes it only work on Mac systems with homebrew. I think this is a shame for Linux users since the guide on the site might be outdated and be missing information that would be useful for everyone. The README says that asciidoctor might be brought in place of asciidoc, but I think this should work before that time. This PR checks to see if the host is Mac and selects a different directory based on whether we've on a Mac or not. 

I've tested this in Ubuntu LTS 12.04 and Fedora 19, and they've both worked great. I've updated the README to include setup instructions on how to install dependencies and setup. There was an odd quirk in Ubuntu that required me to uncomment a line in the asciidoc.conf file before it would observe the source-highlighter attribute. That line is notably absent from the default Fedora 19 conf file included in the RPM and the conf file provided by homebrew.

A stab at adding support for MacPorts users is also included. I suspect it will work as MacPorts strongly discouraged changing your install directory unless your a MacPorts developer, but I haven't tested it yet. I'm not a MacPorts user, so I'll have to set it up to give it a shot. I'm submitting the pull request before testing just in case a MacPorts users comes by with some time to kill. :wink: