Update csharp-mode package to maintained copy on Github,

This update replaces the un-maintained version found on EmacsWiki
http://www.emacswiki.org/emacs/csharp-mode.el

This version was originally based on the following version found on
Google code, which has been 100% un-maintained since 2011.
https://code.google.com/p/csharpmode/

Original author has been contacted via several means, but no reply
has been received.
Merge pull request #2225 from josteink/master

Update csharp-mode package to maintained copy on Github