Fixed next_invoice_date calculation on groups.

The calculation now takes only contracts in active states (waiting + active)
and is recalculated if contracts change state or next_invoice_date.
Bug fix on next_invoice_date calculation.
[ADD] Project info retrieving
[ADD] Community prototypes for value retrieval for description generation.
[ADD] Access rights on child_property and child_property_value
[ADD] Compassion countries. [FIXED] headers
Added Unit Test Files in order to test information retrieval on projects. The tests should be completed as they don't cover everything yet.
Retrieving informations for project description generation
improved description generation
initial german description commit
french description splitted from child description file
improving german description generation
Finalizing german description generation + pep8 corrections
Improving German translation
german description generation improved (80%)
reviewed code corrected
pep8 errors corrected
Pep8 + code reviewed corrected
bug line 309-310, project_code -> None
bug line 301-302, project_code -> None