Define xrange() for Python 3
Define xrange() for Python 3

flake8 testing of https://github.com/google/styleguide on Python 3.6.3

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./cpplint/cpplint.py:1526:12: F821 undefined name 'xrange'
  for i in xrange(startpos, len(line)):
           ^
./cpplint/cpplint.py:1755:15: F821 undefined name 'xrange'
  for line in xrange(1, min(len(lines), 11)):
              ^
./cpplint/cpplint.py:1957:12: F821 undefined name 'xrange'
  for i in xrange(1, len(raw_lines) - 1):
           ^
./cpplint/cpplint.py:2298:14: F821 undefined name 'xrange'
    for i in xrange(linenum - 1, self.starting_linenum, -1):
             ^
./cpplint/cpplint.py:3080:26: F821 undefined name 'xrange'
    for start_linenum in xrange(linenum, clean_lines.NumLines()):
                         ^
./cpplint/cpplint.py:3537:14: F821 undefined name 'xrange'
    for i in xrange(first_line, last_line + 1, 1):
             ^
./cpplint/cpplint.py:3601:19: F821 undefined name 'xrange'
    for offset in xrange(endlinenum + 1,
                  ^
./cpplint/cpplint.py:4155:14: F821 undefined name 'xrange'
    for i in xrange(linenum + 1, end_line):
             ^
./cpplint/cpplint.py:4283:23: F821 undefined name 'unicode'
  if isinstance(line, unicode):
                      ^
./cpplint/cpplint.py:4928:12: F821 undefined name 'xrange'
  for i in xrange(linenum, max(-1, linenum - 10), -1):
           ^
./cpplint/cpplint.py:4949:12: F821 undefined name 'xrange'
  for i in xrange(linenum, max(-1, linenum - 10), -1):
           ^
./cpplint/cpplint.py:4965:12: F821 undefined name 'xrange'
  for i in xrange(linenum, 1, -1):
           ^
./cpplint/cpplint.py:5066:20: F821 undefined name 'xrange'
          for i in xrange(startline, linenum + 1):
                   ^
./cpplint/cpplint.py:5090:14: F821 undefined name 'xrange'
    for i in xrange(linenum - 1, max(0, linenum - 10), -1):
             ^
./cpplint/cpplint.py:5121:14: F821 undefined name 'xrange'
    for i in xrange(2):
             ^
./cpplint/cpplint.py:5284:14: F821 undefined name 'xrange'
    for i in xrange(linenum - 1, max(0, linenum - 5), -1):
             ^
./cpplint/cpplint.py:5503:18: F821 undefined name 'xrange'
  for linenum in xrange(clean_lines.NumLines()):
                 ^
./cpplint/cpplint.py:5639:21: F821 undefined name 'xrange'
  for start_line in xrange(linenum, min(linenum + 3, clean_lines.NumLines())):
                    ^
./cpplint/cpplint.py:5654:12: F821 undefined name 'xrange'
  for i in xrange(end_line, min(end_line + 3, clean_lines.NumLines())):
           ^
./cpplint/cpplint.py:5911:15: F821 undefined name 'xrange'
  for line in xrange(clean_lines.NumLines()):
              ^
```
google/styleguide,google/styleguide,google/styleguide,theandrewdavis/cpplint,google/styleguide