added option "rpushbullet.defdevice" to indicate the default device
  index
added .getDefDevice(), similar to .getDevices()
updated pbPost() to handle vectors/lists of device indices as well as
  default devices and all (pushbullet) devices
updated documentation
Merge pull request #3 from r2evans/defdevice

vectorized deviceind, default device, and all devices
minor tweak to Bill's pull request: prefer 'defaultdevice' as variable
pbGetDevices() is now a proper S3 method
default device behaviour in pbPost was changed

new default is to send to all
name matching also supported
a missing backslash, and a correction to init

set default device to zero in init as well