Improved regex for SQL group, order, from

This is an improvement of `Zend_Db_Select` regarding the SQL regular expressions used in `oder()`, `group()` and `from()` functions. This fix was based on to the feedback received in  `zf-security ` mailing list, about the [ZF2014-04](http://framework.zend.com/security/advisory/ZF2014-04).  According with the reporters, this is an application issue rather than an framework issue. That's why we are not doing a security advisory for it.