Making AssertTextFilesEqualTests work on Windows

NamedTempFiles cannot be read under Windows. This change switches to using osutils.mktemp.
exposing PyIO so subclasses can provide custom behavior

needed because CCP has custom subclasses to handle yaml serialization
Merge pull request #6 from ccpgames/gaggle_AssertTextFilesEqualTests_work_under_windows

AssertTextFilesEqualTests work under windows
Merge pull request #7 from ccpgames/gaggle_exposing_pyio

exposing PyIO so subclasses can provide custom behavior