Fix minor typos and update marketing copy.

Correct spelling mistakes in the AUTHORS and DISCLAIMER files, and
update the README.markdown file to credit Illumos and mention that
the ZPL is finished.

The README.markdown file is also the first impression for a handful
of new users that discover ZoL through a web search because it
doubles as the splash page for the Github repository. The build blurbs
are therefore removed because these people should be encouraged to
visit the regular home page before installing the product.