pbSetup: load token

[csgillespie]

On the initial set-up using pbSetup() the key is forgotten.

• Proposal: after running pbSetup() load the key using the same mechanism as library(RPushbullet) uses
  Happy to submit a PR

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Example:

library(RPushbullet)
#Attaching RPushbullet version 0.3.1.
# No file ~/.rpushbullet.json found. Consider
pbSetup("XXX")
pbPost("note", "test")
# Error: Neither option 'rpushbullet.key' nor entry in package environment found. Aborting

Restarting R

library(RPushbullet)
pbPost("note", "test")

now works

[eddelbuettel]

Interesting. Clearly an overisght.

A nice little PR (including a ChangeLog entry) would be great!