Allow model to set table_name after acts_as_paranoid macro

[AlexWheeler]

I originally opened #117 without tests, and since then have deleted my fork, so reforked and opened this PR with a spec @mvz

Thought about also asserting that ParanoidWithExplicitTableNameAfterMacro.only_deleted is working since that's how I found the error, but I think this test is clearer what exactly we're testing. Lemme know what ya think

• paranoid_column_reference is currently a class attribute which is set
  when a class is loaded and the acts_as_paranoid method is invoked. A
  rails model can set its table_name after it declares itself as
  acts_as_paranoid. This means that whenever paranoid_column_reference is
  referenced in the gem it does not match the table_name for the model
  which can cause errors.

• e.g. ActsAsParanoid::Core#only_deleted references
  paranoid_column_reference so in the below examples when we call this
  method on the paranoid class it tries to look up a table_name matching
  the model name. If the table doesn't exist a sql error is raised because
  the table does not exist. If the table does exist (maybe you're moving
  data to a new table and deprecating old ones) you'll end up returning
  the old records.

• This change makes it a method so that it returns whatever the current
  value of table_name is

  # current incorrect behavior
    class User
      acts_as_paranoid
      self.table_name = "Account"
    end

    User.paranoid_column_reference
    => "user.deleted_at"

   # new correct behavior
    class User
      acts_as_paranoid
      self.table_name = "Account"
    end

    User.paranoid_column_reference
    => "account.deleted_at"

[mvz]

Thanks, @AlexWheeler!