Add fetch, pull, push operation on git-notes

[DrSensor]

g-u-c/guc-desktop@abac4cd

These operations are a bit different with others git-notes operation because it doesn't require handler. Some ways to use the API that I can think of:

notes({ ref: 'utopian' }).push() // or pull() | fetch()
notes({ ref: '*' }).fetch() // or pull() | push()

notes({ ref: '*', remote: 'origin' }).fetch() // or pull() | push()

notes({ref: 'utopian'}).atBranch('master').fetch(...flags) // or pull() | fetch()
notes({ref: 'utopian'}).atRemote('origin').fetch(...flags) // or pull() | fetch()

notes({ref: 'utopian'}).at({ remote: 'origin', branch: 'master' }).push(...flags) // or pull() | fetch()

notes({ref: 'utopian'}).at('origin', 'master').push(...flags) // or pull() | fetch()

Which one is better?