merge changes from the backend-work-1 branch.

[Linas Vepstas]

Log Message:
-----------
merge changes from the backend-work-1 branch.
These changes conisted of:
-- change the backend begin()/commit() signatures to take qofinstance
   instead of gpointer
-- clean up non-standard use of books by the pricedb 
-- rename the gnc-be-utils.h file so that its now a part of QOF

Modified Files:
--------------
    gnucash/src/backend/file:
        gnc-backend-file.c
        io-gncbin-r.c
        io-gncxml-v1.c
        io-gncxml-v2.c
    gnucash/src/backend/postgres:
        PostgresBackend.c
        book.c
        gncquery.c
        price.c
        putil.h
    gnucash/src/business/business-core:
        gncBillTerm.c
        gncCustomer.c
        gncEmployee.c
        gncEntry.c
        gncInvoice.c
        gncJob.c
        gncOrder.c
        gncTaxTable.c
        gncVendor.c
    gnucash/src/engine:
        Account.c
        Group.h
        Makefile.am
        Period.c
        Transaction.c
        gnc-pricedb-p.h
        gnc-pricedb.c
        gnc-pricedb.h
        qofbackend-p.h
        qofbook-p.h
        qofbook.c
        qofbook.h
        qofinstance-p.h
        qofinstance.c
        qofquery.h

Added Files:
-----------
    gnucash/src/engine:
        qof-be-utils.h

Removed Files:
-------------
    gnucash/src/engine:
        gnc-be-utils.h