= New Features

* Model#get_column_value and #set_column_value have been added for
  getting/setting column values.  Historically, to get column
  values, you would just send the column name, and to set column
  values you would send the column name suffixed by =.  However,
  this doesn't work when such methods are already defined by
  ruby or Sequel itself (e.g. class, model, object_id).

  Both #get_column_value and #set_column_value are just aliases to
  #send, but you can safely override the methods to handle column
  names that conflict with existing method names.  Both the core
  model code and all of the plugins that ship with Sequel have
  been updated to use these new methods.  External plugins are
  strongly encouraged to switch to these new methods.

* A column_conflicts plugin has been added to automatically handle
  columns that conflict with existing method names.  So if you
  have a column named "model" in your table, you can just load the
  column_conflicts plugin and Sequel will handle things correctly.

* A accessed_columns plugin has been added, which records which
  columns have been accessed for a model instance.  This is useful
  in development when you are planning on restricted the columns
  selected by the dataset that retrieved the instance.  SELECTing
  only the columns you need can result in significant performance
  increases, and the accessed_columns plugin makes that easier.

* Model#cancel_action has been added for canceling actions in
  before hooks, instead of having the before hook methods return
  false (which is still supported).  In addition to being easier to
  use, this also makes it possible to use custom exception messages
  for hook failures, if you are using the default behavior of raising
  exceptions on save failures.

= Other Improvements

* Dataset#union, #intersect, and #except now automatically handle
  datasets with raw SQL, by wrapping such datasets in subqueries.

* The integer migrator now stores the correct migration number
  when migrating with allow_missing_migration_files set.

* A :timeout=>nil Database option on MySQL no longer sets a
  wait_timeout.
