Add #mark_as_<verb> and #mark_as_not_<verb> instance methods

These methods allow us to set values without saving the record. We’re now able to do this :

```ruby
class User < ActiveRecord::Base
  acts_as_microscope
end

@user = User.first
@user.mark_as_blocked
@user.blocked? # => true
@user.blocked_at # => 2014-05-23 09:04:35 -0400
@user.reload
@user.blocked? # => false
@user.blocked_at # => nil

# In contrast to:

@user = User.first
@user.block!
@user.blocked? # => true
@user.blocked_at # => 2014-05-23 09:04:35 -0400
@user.reload
@user.blocked? # => true
@user.blocked_at # => 2014-05-23 09:04:35 -0400
```