Review of std.signal

[Robert]

I changed the string mixin now to take an enum instead of the 
string for specifying the protection. You convinced me, it is 
just nicer.

  https://vhios.dyndns.org/dlang.org/web/phobos/std_signal.html
  
https://github.com/phobos-x/phobosx/blob/master/source/phobosx/signal.d

I will also look into improving documentation a bit soon.

Best regards,
Robert

> Robert, it will be great to see some more documentation for 
> `std.signal`.
> For example, you can:
>  - add more usage examples for each public function
>  - add tread-safe example for multi-tread application
>  - add better signal description and add more external links 
> (for examle, to the wikipedia - 
> http://en.wikipedia.org/wiki/Signals_and_slots). Look at the 
> old std.signals - http://dlang.org/phobos/std_signals.html