Fixed header levels in markdown

Fixed header markup in README.md. Previously, header markup did not match levels in the content list.
Anchors and reorder in dependency lists of README.md

Dependency headers for OSes needed anchors to work with links from
content list. Reordered OSes in content list to match headers
(all Linuxes first, Mac OS X last)
Merge pull request #180 from pmandera/readme-links

Fixed headers and links to dependencies in README.md