Add support for more fetch modes.

This patch adds support for the following fetch modes:

  PDO::FETCH_COLUMN
  PDO::FETCH_OBJ
  PDO::FETCH_INTO
  PDO::FETCH_CLASS
  PDO::FETCH_CLASS | PDO::FETCH_PROPS_LATE

In order to add these new fetch modes, Statement::setFetchMode() was
implemented, and all other functions that had arguments for fetch mode
have now been implemented.

This patch also fixes a bug in Statement::fetchColumn where it would only
return the first column, even if colNumber was set.