This alarm send the first invoice reminder to the user.

This will be handle by global alarm to check the user status, and not invoice
by invoice.

It allows to have understandable accounting date on the invoice.

Should be completely refactored in future to something less random.

Otherwise, periodicity may force processing the instance, but it will fail, BUT the timestamp file is still present, so it won't be processed thereafter.

Also include test updates demonstrating behavior change.

Use http://tools.ietf.org/html/draft-kelly-json-hal-05 has base format.
Provide possibility to request instance, browse them and check their message
and connection parameters.

If SR is not correctly installed, slapgrid complained that "file already exists". This is misleading error.