[WIP] Cubrid db support

fixes #36 

WIP, do not merge yet

Open issues:
- [x] Bug in CUBRID PDO: http://jira.cubrid.org/browse/APIS-658 https://gist.github.com/cebe/6461443
- [x] complete unit tests
- [x] support composite foreign keys
- [x] correct handling of boolean type
- [x] correct handling of blob and clob types