Reuse token during one simplenote-sync-notes process to make sync faster

Hello @dotemacs 

Currently _simplenote-sync-notes_ calls _simplenote-token_ to get a token for syncing each note, but don't I think it's necessary to get a token for syncing each note. Because once getting the token, it's valid as far as keeping access to the site within a certain period of time.

I modified _simplenote-sync-notes_ so it reuses the token in one process. This change can make _simplenote-sync-notes_ faster when you need to sync many notes. The following results are my case in which I download 156 notes.

- Before: 2:31
- After: 1:10