Updating Ubuntu docs to omit "ruby dev tools"
Adding note that Windows DevKit compilation is not supported.
Cleanup on the OSX section.
Updating RedHat section.
Editing the "nonstandard libxml2 installation" section.
Adding a section on "Using your system libraries"
Reordering sections.
DRYing up URLs
Cleaned up preamble.
Cleaning up "nonstandard" section.