cli: add script to register hook

Could easily be extended to register a hook on more than one repo at a
time, looping over all repos editable by the given user, or taking in
a list.
travis-ci: drop node 0.6 testing

The tests were passing, but 'npm install' couldn't connect via https
cause the VM the tests were running on doesn't have the right Cert
(or something similar)
README: update for register-hooks script
Typo in log message
Merge subscribe-to-notifications (#4) into master

cli: add script to register hook

Could easily be extended to register a hook on more than one repo at a
time, looping over all repos editable by the given user, or taking in
a list.