Fixed XLS import on python 3. Optimized loop

Fixed XLS import. Changed xrange function to range for use with python3.
Throw out from loop unneeded "if" statement.
Used SIX library way to express the range.
Merge pull request #120 from frank-u/master

Fixed XLS import on python 3. Optimized loop
Add frank-u to AUTHORS
Merge branch 'master' of https://github.com/bmihelac/django-import-export

* 'master' of https://github.com/bmihelac/django-import-export:
  Add frank-u to AUTHORS
  Used SIX library way to express the range.
  Fixed XLS import on python 3. Optimized loop