Added header image

git-svn-id: http://plugins.svn.wordpress.org/black-studio-tinymce-widget/assets@579690 b8457f37-d9ea-0310-8a92-e5e31aec5664
Moved screenshots to assets folder
Added .git* files
added icons and updated screenshots
updated .gitignore
Merge branch 'assets' into develop
updated screenshot links