compat: add pptx.compat sub-package
compat: change XmlString to inherit from Unicode
compat: make EnumerationBase Python 3 compatible
compat: replace StringIO with BytesIO
compat: refactor dml.color._Color to Python 3
compat: refactor mock usage for Python 3
compat: fix string usage for Python 3
compat: fix BaseOxmlElement metaclass for Python 3

* also retire dead code in oxml.xmlchemy
compat: fix Python 3 float difference

series_values come out of count as 1.1, 2.2, 3.300000003 in Python 3.
compat: fix integer division for Python 3
compat: use is_integer() for Python 3 compatibilty
compat: change file to io.RawIOBase for Python 3
compat: add __bool__ in text.layout for Python 3
compat: use list(range()) for Python 3
compat: remove relative import for Python 3
compat: use BytesIO for Python 3
compat: open files binary for Python 3
compat: refactor __str__ for Python 3
build: add tox.ini config for py33 and py34
docs: add Python 3 support to Installation page