Refactorized and improved codesnippet sample.
Code refactoring of codesnippetgeshi:

* Use writeChildrenHtml.
* Code-style.
* Added sample.
Code refactoring: minor fix in codesnippet dialog.
Removed lib directory from codesnippetgeshi plugin.
Code refactoring: restricted number of languages in codesnippetgeshi sample.
Improved Code Snippet and Code Snippet GeSHi samples.
Fixed: Code Snippet does not work in inline editors due to asynchronous highlighting library load. Introduced CKEDITOR.plugins.codesnippet.highlighter class.
Fixed: Data should not be converted into JSON in CKEDITOR.ajax.post.
Updated codesnippetgeshi to latest CKEDITOR.ajax.post API.
Removed obsolete Content-type header from CKEDITOR.ajax.post.
Extended CKEDITOR.ajax.post with optional Content-type header. Added docs for the method.
Code refactoring: converted function expressions into function declarations and fixed code indentation in AJAX plugin.
Added undo to sample toolbars.
Fixed updateSnapshot to prevent broken undoManager stack.
Code refactoring: code style in contents.css.
Code refactoring: config variable in camelCase.
Moved documentation of Code Snippet GeSHi to README file.
Added reference to Code Snippet GeSHi in Code Snippet sample.
Fixed: Don\'t load hljs in IE8.
Register CodeSnippet button in plugin's init.