Mark as available and notify all users whose 'become available' date passed

As a cron job, mark as available to host all users whose 'become available'
date has passed, and notify them via email of this change. RE: Issue #309
Move availability update within notification loop, check become available <> 0

Move the update query within the loop that notifies each user.  Add an
additional check that become available is not equal to zero, because the
way hook_update_N() with a db_add_field() works results in everything
being null, but new items could be created with 0. Remove unnecessary
static query parameters. RE: Issue #309 and Pull Request #370.
Merge pull request #370 from dmurphy1/20140102_process_users_that_become_available

Mark as available and notify all users whose 'become available' date passed