Convert print statements to print functions
Migrate from forward-incompatible iterator methods
Use Unicode strings throughout library

Python 3 will require it, so also do it in Python 2 for consistency.
Simplify slugify implementations
Use io.BytesIO instead of StringIO

On Python 3, StringIO is for strings.
Ensure _OpenSlideMap.keys() returns view object on Python 3

Make the existing keys() method private to avoid overriding
Mapping.keys().
setup.py: Update syntax for Python 3
README: Document Python 3 support

Closes #3.