explicit require "forwardable"

I was having trouble using the gem in irb:

```ruby
ree-1.8.7-2012.02 :003 > require 'pastel'
NameError: uninitialized constant Pastel::Delegator::Forwardable
```

 The issue was that the "forwardable" library was to be explicitly required. This commit fix the issue :)

Thanks for the gem!

Andrea