Added Ivan Towlson's C# articles with his kind permission.

[soc]

• I left out some Scala-on-.NET specific parts, which should probably
  be reviewed and put into a diferent article.
• I did some minor changes to the wording, I'm a bit reluctant
  to do more extensive changes because Ivan's personal writing style
  make it such a joy to read and English isn't my mother tongue.
• I replaced the notion of “Pimp My Library” with the more neutral
  “Enriching ...” as discussed on the mailing list. I'm open to
  better proposals.
• I replaced “operators” with “methods” if it revolved around the name
  and with “methods in infix notation” if it revolved around the calling syntax.
  Both as mentioned in the style guide.

Some thougths:

• The article was written before C# and Scala got optional/default
  parameters, we probably need to adjust this section
• C# also got Tuples ... not sure if or how much we need to cover it now.
• We could probably need some introduction to syntax differences at the
  beginning.