Add support for C# and Gherkin

In this change set I have two commits each propose addition of a new language.

csharp: extends clike and changes some properties.
gherkin: support for most of the keyword, step, string, etc for gherkin. However I have included a TODO list that other people could possibly look into adding.

Please let me know if you think any part could be improved. and BTW, this is by far my favorite syntax highlighter.