Add a test for multiple simple empty MCOS.

Verify everything continues to work with multiple elements.
When parsing the MAT file header, store the byte order used as well.

For applications parsing the MAT file later, they may need to know the byte
order of the original file.  Thus store that information in the header.  Note
for writing it is possible to change the endian indicator without changing the
byte order returned.  This should eventually throw an exception.
Start parsing the binary blob.  First things first, get the class name.

For the future, the class name will need to be fetched before I know about
the placeholders.  Thus read in the class names and use that now.
Make sure the MCOS header for objects is present.

To avoid future surprises, make sure that the MCOS header bytes for an object
are correct.