using Dictionary for caching causes multi-thread issues.

In RAdapter and FontsHandler, Dictionaries are used to cached brushes, pens and fonts.

Single instance pattern is used by PdfSharpAdapter, and this instance is used by all HtmlContainers generated during system running time.

It causes problem when using this single instance in a multi-thread environment. [Refer Here](http://stackoverflow.com/questions/1920864/when-does-a-dictionary-throw-an-indexoutofrangeexception-on-add-or-containskey)

Please replace Dictionary with ConcurrentDictionary.