addded before/after db cleanup methods to allow for vendor specific behaviour

Conflicts:
	build.xml
	src/de/fuberlin/wiwiss/d2rq/sql/vendor/SQL92.java
	src/org/d2rq/download/DownloadContentQuery.java
merged cherry-picked 6d138879c65e104e77bff46b6803ffbf2593de06 from master
fixed before/afterExecuteQuery
Merge pull request #243 from syapse/develop-postgresql

Provide before() after() hooks for statement query/close/cancel - cherry-picked