#76 Implements weasyprint to generate invoice pdf

Fixes https://code.ffdn.org/FFDN/coin/issues/61

== WIP ==

[ ] Add information about Debian dependencies in README (libffi-dev libxml2-dev libxslt1-dev)

[ ] Handle case where no offer reference are defined

+            {% if detail.offersubscription.offer.reference %} ({{ detail.offersubscription.get_subscription_reference }}){% endif %}

^ So I guess "[] Handle case where no offer reference are defined" is ok ?

I guess you're right

added tests, cleaned history & rebased against master. I let you testing a last time & merging @opi :-)

thanks everyone for the tremendous work.