Database access layer, small fixes:
	link available soci backends (for helpers)
	remove 3 ctors from database.cpp, implementation will be moved/splitted in future
	add db::serial type for IDs
	add super_t typedef to entities, to simplify entity ctors
	fixed (again) iterator for selects, now it can handle operators ->, x++ and ++x
Database access layer, new feature:
	insert_statement: helper for inserting whole rows
	backend helpers, currently only sqlite3 and only last_insert_id()
	query_builder - build simple (aka full insert/update) statements easy

To all above: test case added & tests updated.