Started to eliminate global assignments just cause by getAuth

This is the  message according to R's build check.. just added the relevant part here:

File 'RAdwords/R/getAuth.R':
  assign("credentials", credentials, envir = .GlobalEnv)
  assign("saveCred", saveCred, envir = .GlobalEnv)
File 'RAdwords/R/loadToken.R':
  assign("access.token.timeStamp", as.numeric(Sys.time()), envir = .GlobalEnv)
  assign("access.token", a, envir = .GlobalEnv)
File 'RAdwords/R/refreshToken.R':
  assign("access.token.timeStamp", as.numeric(Sys.time()), envir = .GlobalEnv)
  assign("access.token", access.token, envir = .GlobalEnv)


I suggest what I did in the code example. obviously a few files need to be adapted to using environment credentials_env.