Fix printing classes/instances to the console, add trailing \n

Priting in RStudio was fine, because it probably adds a newline
as needed before printing the prompt. On the console a final
trailing newline was missing, which was annoying, because the
prompt appeared right at the end of the last line of the printout.
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 pull request #7 from gaborcsardi/fix/trailingnewline

Fix printing classes/instances to the console, add trailing \n
Add tests for inherited method environment
Add information about fields with reference objects
Simplify NULL/0-length check
Update NEWS
Merge branch 'feature/print' of github.com:gaborcsardi/R6

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