Fix unicode encode errors on Python 2 -- Fixes #215

Switch csv library to backports.csv as the implementation
is closer to the python 3 one. Add a test case covering the
problem.

Run tests with unicode_literals from future

Fix unicode encode errors with unicode characters

- Use `backports.csv` instead of `unicodecsv`
- Use StringIO instead of cStringIO
- Clean-up some Python 2 specific code

setup.py

@@ -44,7 +44,7 @@ packages = [
 install = [
     'odfpy',
     'openpyxl',
-    'unicodecsv',
+    'backports.csv',
     'xlrd',
     'xlwt',
     'pyyaml',