RC_* to RINGCENTRAL_*
APP_KEY and APP_SECRET repalced with CLIENT_ID and CLIENT_SECRET to
reflect RingCentral's descriptions
Change RC_SANDBOX to RINGCENTRAL_SERVER and changed its behaviour.
Allows setting the maximum age of voicemails to fetch on the first run.
Other changes:
- Refactor `from` to `age` in function parameters
- Improve README
