Add ability to document model

For example:

/** 
  *  A train route defines a code and a carriage count.  Stops on the route are 
  *   defined elsewhere. 
  */
CREATE TABLE train_route (

  /** The auto generated primary key for this table. */
  _id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,

  /** The unique code for this route, e.g. P1231X */
  train_code TEXT NOT NULL,

  /** The number of carriages on this route. */
  carriage_count INTEGER NOT NULL

);

Would produce:

/** 
  *  A train route defines a code and a carriage count.  Stops on the route are 
  *   defined elsewhere. 
  */
public interface TrainRouteModel {
  String TABLE_NAME = "train_route";

  String _ID = "_id";

  String TRAIN_CODE = "train_code";

  String CARRIAGE_COUNT = "carriage_count";

  String CREATE_TABLE = ""
      + "CREATE TABLE train_route (\n"
      + "\n"
      + "  _id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,\n"
      + "\n"
      + "\n"
      + "  train_code TEXT NOT NULL,\n"
      + "\n"
      + "  carriage_count INTEGER NOT NULL\n"
      + "\n"
      + ")";

  /**
   * Find a train route by its code.
   */
  String SELECT_BY_TRAIN_CODE = ""
      + "SELECT *\n"
      + "FROM train_route\n"
      + "WHERE train_code = ?";

  /** The auto generated primary key for this table. */  
  long _id();

   /** The unique code for this route, e.g. P1231X */
  @NonNull
  String train_code();

  /** The number of carriages on this route. */
  long carriage_count();

...

[JakeWharton]

I would also hoist those comments onto the column constants.

[AlecStrong]

looks perfect, open a PR and we can continue from there