Revision 150605

Add a command-line option which permits the norms from a single field
to be used for all fields.  Norms are read into memory, using a byte
of memory per document per searched field.  This can cause search of
large collections with a large number of fields to run out of memory.
If all of the fields contain only a single token, then the norms are
all identical, and a single norm vector may be shared.