python-bindings: improvements to str_methods

[Christoph Holtermann]

Hello !

I added some code:

str_methods.py

- added a flagging system to change the output of the __str__ methods
- added unicode and str-methods to invoice and entry

function_class.py
- added modified add_method and add_classmethods to implement the previous

gnucash_business.py
- add methods_return_instance_lists for method Invoice.GetEntries

The patch is [1] for "bug" [2].

regards,

Christoph Holtermann

[1] https://bugzilla.gnome.org/attachment.cgi?id=188696&action=diff
[2] https://bugzilla.gnome.org/show_bug.cgi?id=639906