Add SourcePawn language

SourcePawn is the language used for writing [SourceMod](http://www.sourcemod.net/about.php "SourceMod") plugins that modify games running on Valve's Half-Life 2 engine.

This language is becoming very popular on Github as can be seen [here](https://github.com/search?q=%23include+%3Csourcemod%3E&type=Code&ref=searchresults) and [here](https://github.com/search?q=if+extension%3Asp&type=Code&ref=searchresults).

This pull requests includes changes requested in my previous pull request #497.