Added the 1.3 release to the changelog and upgrade notices.

git-svn-id: https://plugins.svn.wordpress.org/latex-everything/trunk@836784 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added a more informative error message when pdflatex and pdftk are not found.

git-svn-id: https://plugins.svn.wordpress.org/latex-everything/trunk@836785 b8457f37-d9ea-0310-8a92-e5e31aec5664
Tested up to Wordpress 3.8.0.

git-svn-id: https://plugins.svn.wordpress.org/latex-everything/trunk@836786 b8457f37-d9ea-0310-8a92-e5e31aec5664
Added notes to readme.txt that the plugin is unsupported.

git-svn-id: https://plugins.svn.wordpress.org/latex-everything/trunk@836787 b8457f37-d9ea-0310-8a92-e5e31aec5664