vectorized deviceind, default device, and all devices

This fork/branch suggests three additions to the code:

1. Vectorize (via `c` or `list`) the `deviceind` for `pbPost()` in order to send the same message to multiple devices; this is handled simply with `sapply` since I did not find in the pushbullet API the ability to push to multiple (but not all) devices in one call.

2. Define a default device instead of default to 1. This can be set in the same manner as `devices`, using a key `defdevice` in the usual places.

3. Allow for sending to **all** devices (as listed in the pushbullet account) instead of one or a subset. This deviates a little from the inferred subset as provided in `rpushbullet.devices`.