Add a few more alias methods

[andrewhavens]

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:

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

This avoids having to write something like:

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