invoke doesn't handle multiple classes using the same namespace

Simplified example. This doesn't work but should.

``` ruby
class A < Thor
   namespace :stuff
  desc 'a'; def a; end
end

class B < Thor
   namespace :stuff
  desc 'b'; def b; end
end

class C < Thor
  desc 'abc'; def abc
    invoke 'stuff:a'
    invoke 'stuff:b'       # Says b() not defined
  end
end
```
