Implement initial command permissions system

After our initial discussion on the matter (see #172) this is the system
that we all seem to have agreed on. As a result, I have implemented a
simple system which effectively implements permissions, while being
extensible and tweakable so bot devs can decide what they want to do for
permissions.

As for default 'permissions', I'm not sure what the best approach would be
here; bot devs are likely to implement their own permissions 'levels' and
use those. I think the most we could do for now is add attributes to
require certain users (by id) and certain roles (by id and possibly by
name?) This would probably be the best option for now as it requires less
work from us, nor do we know the *exact* approach bot devs want to take
with permissions.

src/Discord.Net.Commands/CommandError.cs

@@ -16,5 +16,6 @@
 
         //Execute
         Exception,
+		InvalidPermissions
     }
 }