Fixing support for unicode strings under Python 2.x in ArrowFactory.get

isinstance(arg, str) will not match unicode strings under Python 2.x

I've added an isstr() function to arrow.util which will correctly match all string types under Python 2.x and 3.x. 

Using six is another possibility. It's not yet a direct dependency in arrow, but it is in dateutil anyway.

See http://stackoverflow.com/questions/11301138/how-to-check-if-variable-is-string-with-python-2-and-3-compatibility for a discussion on the problem.