Reverted 0bf2dadd: it's very common to use lower case for column types and upper case for other DB keywords.

` is a MySQL thing supported by cubrid. " is more common.

http://jira.cubrid.org/browse/APIS-658

bit has special syntax for storing and retreiving so we'd need a
converter for that.
Storing 1 bit will result in one byte left padded on the disc so the
result of a query for boolean 0 will be 0x00 and for boolean 1 will be
0x80.

this allows different implementation in different DBMS

CUBRID does not supprt PDO::TYPE_BOOL, so we use STRING here which will
be casted by the DBMS