Finish windows and mac instructions.

* Cleaned up some of the links.
* Created a more structured installation instructions section.
Make homepage link more descriptive.
Improve wording about master branch.
Fix internal links.
Play around with headings.
Clean up Mac code block.
Remove broken link ref, add '/' to a path.
Done with apt-get instructions.
UNIX 'get the source' instructions.
Write the UNIX build instructions.
Playing around with code blocks.
Finish UNIX instructions.
Touch up comments regarding lib path.
Miscellaneous fixes.
Miscellaneous fixes.
Improve code block aesthetics.
UNIX installation layout.
Remove 2 periods for consistency.
Add SIMBODY_HOME instructions.
Fix language about x86_64...