Feature/all privileges

Lets you do
```ruby
@policy_machine.scoped_privileges(@user,@object)
```
and get a result formatted like the result of `@policy_machine.privileges`. Unlike that method, it also accepts user attributes and object attributes. Not sure about the name.

This seems like as good a time as any to start maintaining the changelog. If this is merged, I'll push the gem to rubygems as version 0.0.2.

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