Large rework of PayPal import, improved scriptlet expressiveness

With the now much larger pool of different PayPal transactions
available, reworked pygnclib's paypal.py and what scriptlets can
actually do. Some default actions are now controlled by file-global
variables:

merge_nextline - when true, join with next line, no import
store_fwdref   - when true, stash away and later join with
                 referenced transaction (not sporting a ref txn
                 is an error)
ignore         - simply ignore this transaction

The actual importer function is now called with some keyword args,
to cut down on arg proliferation.