Update README with mo betta file opening examples

Use contexts for file opening, along with reading / writing in binary
mode for MARC21 binary files.

Also use the print method with parentheses for broader compatibility.

Signed-off-by: Dan Scott <dan@coffeecode.net>
README examples: open() mode and iterator

Thanks to Geoffrey Spear for catching the open() mode requirement. Also, change
the iterator to next(reader) instead of reader.next().

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #51 from dbs/python3_dbs

Update README with mo betta file opening examples