Enumerable#exactly? - more examples, more specs

Hey @bbatsov, I noticed that one of the examples for `Enumerable#exactly?` was incorrect, so I fixed it in a way that I hope you originally intended.

In the process, I have also added some more examples, as well as corresponding specs, to better document the behaviour of `Enumerable#exactly?` in the absence of a block.

Thanks for considering this pull request... great little library, BTW!  :smile:

---

Here's what the example resulted in, due to the lack of an argument:

```ruby
irb(main):001:0> require 'powerpack'
=> true
irb(main):002:0> [1, false, nil].exactly? #=> false
ArgumentError: wrong number of arguments (0 for 1)
	from /Users/pvdb/Workarea/powerpack/lib/powerpack/enumerable/exactly.rb:21:in `exactly?'
	from (irb):2
	from /usr/local/var/rbenv/versions/2.0.0-p195/bin/irb:12:in `<main>'
irb(main):003:0> _
```