Minor clarifications in oblig text

I have been made aware of a few errors or missing clarifications in the oblig-text, and have fixed these. Below is a list of changes:
 

- [Exercise 1] Added: " (Note: You do not need to map the sequence home_hid_seq, this is only used by PostgreSQL to generate new keys for home(hid).)"

- [Exercise 3] The Make-rule all_data is not present (anymore) in the Makefile, so I replaced it with "make direct_mappings urb_ontology organization". If you want to add it, simply add the following rule to your Makefile:

all_data: direct_mappings urb_ontology organizations

- [Exercise 4] Added: "Put the rules in semantics/urb_rules.lore."

These changes do not change the exercises in any way, only clarifications. Please feel free to ask if you have any questions :)

- Leif Harald