Users users model - postgresql (case sensitive DDL)

this PR fix two issue (a, b)
### Steps to reproduce the issue 

(a) On administration side
go to User -> User Manager and click to sort for Last Visit Date or Registration Date column

(b) On administration side
go to User -> User Manager and click on search tools and filter on Registration Date

### Expected result

(a) The user list is sorted by registration or visit date
(b) The user list is filtered by register date

### Actual result

(a) but it throws a Database Error
query failed (error # %s): %s SQL=SELECT a.* FROM "#_users" AS a ORDER BY a.registerDate ASC LIMIT 20

(b)but it throws a Database Error
query failed (error # %s): %s SQL=SELECT a.* FROM "#_users" AS a WHERE a.registerDate >= '2014-09-15 19:43:46' AND a.registerDate <='2014-12-15 19:43:46' ORDER BY a.name asc LIMIT 20

### System information

Joomla! 3.4.0-alpha
Postgres 9.3.5
PHP 5.5.9

### Additional comments

Postgres is a little bit different than mysql on mixed case columns name , so this is is only a temporary fix
should be better if all joomla fields became lowercase, the user table is the only table that have mixed case fields