Add a few more alias methods

Adding this as a TODO for myself (or anyone else who wants to contribute).
1. Add `able_to?` method.
2. Add `cannot?` and `not_` (i.e. `not_able_to?`) methods. For example:

``` ruby
if cannot? :view, secret_message
if not_permitted_to? :view, secret_message
```

This avoids having to write something like:

``` ruby
unless can? :view, secret_message
# or...
if !can? :view, secret_message
```
