Python invoice export. Export from templates using jinja2.

This adds a new script gncinvoice_jinja.py and some new templates. 
latex_invoices.py has some additions.

Querying of invoices is done via the querying api provided by Tom Lofts work for a REST Api.

gncinvoice_jinja.py creates invoices from templates using the jinja2 templating framework. This approach is much more flexible then the on used in latex_invoices.py. Two LaTeX templates provided. One using rechnung.sty as did latex_invoices.py, the second using a raw table and the formatting is up to the template.

doxygen docs for the invoice export included.