all.snippets: bbox and box snippet Python 3 compatibility

I'm not much of a python pro, but it appears the correct
way to iterate is next(i) instead of i.next().  i.next() has
been renamed to i.__next__() on python3, but next(i) works on both.

Pasted below is the following test script ran through python
2.6, 2.7, and 3.3:
i = iter("1,2".split(","))
next(i)
i.next()
i.__next__()
next(i)

Python 2.6.8 (unknown, Jan 26 2013, 14:35:25)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> i = iter("1,2".split(","))
>>> next(i)
'1'
>>> i.next()
'2'
>>> i.__next__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'listiterator' object has no attribute '__next__'
>>> next(i)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>>

Python 2.7.6 (default, Dec  6 2013, 20:05:37)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> i = iter("1,2".split(","))
>>> next(i)
'1'
>>> i.next()
'2'
>>> i.__next__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'listiterator' object has no attribute '__next__'
>>> next(i)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>>

Python 3.3.3 (default, Jan  2 2014, 19:09:02)
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> i = iter("1,2".split(","))
>>> next(i)
'1'
>>> i.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'list_iterator' object has no attribute 'next'
>>> i.__next__()
'2'
>>> next(i)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
>>>

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>