Print method for displaying instances on the screen

The public print() method of an instance is called, if present,
from print.R6, the generic R6 print method. This way classes can
define how they want their instances  printed.
Add docs for R6 print method
Add NEWS entry for print method
Add example about print method to Intro vignette
Merge branch 'feature/print' of github.com:gaborcsardi/R6

Conflicts:
	NEWS.md
	R/print.R
	tests/testthat/test-r6-portable.R
	vignettes/Introduction.Rmd