Memoization

See https://github.com/nathanl/authority/issues/52

This is my proof of concept implementation of memoization for Authority (using the [memoist](https://github.com/matthewrudy/memoist) gem since ActiveSupport has removed memoization). It is opt-in via a new configuration parameter, `config.memoization = true`.

My Rails application renders a large number of resources and checks lots of permissions on these resources to determine whether to show various attributes. This change improves the rendering performance of my app by ~100%.

The following methods get memoized:
 * The `authorizer` instance on each resource model.
 * The "adjective" instance methods on each resource model.
 * The "adjective" instance methods in every Authorizer class.
 * The "verb" methods on the user model.

The cache can be reset on each of these classes by calling `#flush_authority_cache`. You can also reset a specific method by passing in `true` as the last parameter (i.e. `user.can_read?(resource, true)`). (See the memoist readme for more details on the interface.)

If this looks good, I'll go ahead and add specs and readme documentation.

Thanks!