Added c interop package to godropbox

This allows a multithreaded C or C++ program to call a goroutine and multiplexes the input and output streams
I'm a bit new to the code review interface here on github: are you referring to this
https://code.google.com/p/go/issues/detail?id=9096

It seems that the maintainers of golang don't want us to loop on writes... am I misreading here?