Some additions to the tutorial for opening files

The tutorial opens a file without closing it.
It's bad for memory management and also introduce another form to write cleaner (for reading) and safer (for memory management)
You can see further on the with statement introduced at the end of the official part https://docs.python.org/2/tutorial/inputoutput.html#methods-of-file-objects
With this PR, the other examples might be changed accordingly.