Mention "print_tree.sql" in README
Add a notice about postgresql 9.3.3 to README
Implement item 9 - "print_tree"

Even though the print_tree function is implemented, it is still needed
to implement text_employee_indented function.
Write print_employee_indented, not working
Rewrite string transformation with proper operator
Reimplement print_employee_indented

Now it takes id instead of table, also now it works. However print_tree
currently does not work.
Fix print_tree (almost completely)

It works if you pass "level_superior_id" as an actual value, but does
not work with NULL.
Fix print_tree to work without level_superior_id

Now it works even if NULL (its default value) is passed.
Merge branch 'item9'