Provide mailbox crud operations and organize module content.

This patch introduces a single function set to read and write mailboxes in database. Currently read/write operations are being done here and there in the module, sometimes as array, others as object.

The functions just mimic the way Drupal 6 and Drupal 7 works with 'entities', however I don't have the plan to convert mailboxes into entities for the D7 version. This functions will make eassier the code review and understanding for anyone used to see _load and _save functions.

There is also a mailbox_defaults function that return an empty mailbox with default values. Currently the default values are the same as the ones used in the adminsitration form, but there is a single change: the format. The module has never used the value FILTER_FORMAT_DEFAULT, because the 'format' form element always selected any of the available formats. For this reason I've changed this constant wit the Drupal site's variable 'filter_default_format' that contains the default format selected by an administrator. (Note that in Drupal 7, format is no longer required by core, and the value of filter is per-field).

This patch introduces the function, but they are only used when manually retrieving the content of a mailbox, because hook_menu, hook_cron, and the administration forms require another patch to make use of them.