Make everyone check for null (public methods) and change PNG so it's
in line with the other parsers.
Removed unneeded using directives.
Small changes and add some text giving thanks to useful resources.
* MinimalWeb.cs: Check input arguments.

* WikipediaTools.cs:
* IvionWebSoft.csproj: Tools for extracting near plaint text from a
  Wikipedia article.
Add Wikipedia support, prints the first 160 characters of any article.