Adds support for Solaris, OpenSolaris, or illumos.

Nokogiri fails to install on Solaris boxes.  This PR fixes this issue.

The reason is that libxslt library will not compile from source without some [tweaks](https://mail.gnome.org/archives/xslt/2011-May/msg00032.html) to the configure script.  The workaround is to either edit the configure script or use an OpenCSW binary.

This PR adds some clarity for Solaris to the installation section of the README, and adds the OpenCSW default paths.