Fixes and improvements to SpooledBytesIO & SpooledStringIO

[bwatson-dk]

• Inherit from io.IOBase so instance & type checks are simpler
• __eq__ and __ne__ no longer need to load both files completely into memory
• __eq__ and __ne__ reset read-head in comparison errors
• Some methods now raise a ValueError if the file is closed. This brings the implementations closer to StringIO and implementations from the io module
• Added some additional unittests
• Removed some Python 2.6 style format strings