Add native sqlsrv and sqlite definitions for date schema fields

Hi,

there is a install-problem located in registration.install

The datatypes of the datetime-fields are declared for mysql and pgsql. The declaration for sqlsrv and sqlite is missing.

The following changes worked for me.

...
'reminder_date' => array(
'description' => 'Date to send the reminder on.',
'mysql_type' => 'datetime',
'pgsql_type' => 'timestamp',
++ 'sqlite_type' => 'varchar',
++ 'sqlsrv_type' => 'smalldatetime',
'not null' => FALSE,
),
...
'open' => array(
'description' => 'Date to open registrations. Or NULL to open immediately.',
'mysql_type' => 'datetime',
'pgsql_type' => 'timestamp',
++ 'sqlite_type' => 'varchar',
++ 'sqlsrv_type' => 'smalldatetime',
'not null' => FALSE,
),
'close' => array(
'description' => 'Date to close registrations. Or NULL to never close automatically.',
'mysql_type' => 'datetime',
'pgsql_type' => 'timestamp',
++ 'sqlite_type' => 'varchar',
++ 'sqlsrv_type' => 'smalldatetime',
'not null' => FALSE,
),