Remove FAT signature check as it fails for Word Documents (.doc) files; requires further investigation.
MSCFBSource: source abstraction for NSData and NSFileHandle
MSCFBFile: switch to use source abstraction
Tests: add test data and some basic unit tests
MSCFBDirectoryEntry: fix mini stream cutoff size check
MSCFBFile: fix off-by-one error reading mini stream
Updated test data.