Remove scanf invocations and improve error messages; uses manual parser now
Remove scanf use from Macaddr; parse '-'-separated MAC addresses

Add ?sep to Macaddr.to_string. Update tests. Use manual parser with better error messages.
Update README; clean-up decimal test in Ipaddr; remove test_ipaddr.ml TODO
Note Macaddr API addition in CHANGES
Merge pull request #8 from mirage/no_scanf

No scanf