CAEA support

[erebodino]

Added CAEA support trying to do the minor changes possibles:

1. Created 3 models to keep track over CAEA: - Caea itself. - InformedCaea ( to keep track over the CAEAs without operations informed to AFIP). - CaeaCounter (to keep track over the number of the receipt created, since this number will be used when AFIP is offline so we must set it before send it).

2. Modify some field over Receipt and ReceiptValidation perhaps is not the best approach but keeps everything very simple to understand.

3. Added a new signal pre_saveattached to Receipt.save() to assign CAEA and receipt_number when necessary.

4. Modified TaxPayer to incorporate new methods to consult/inform CAEA.

5. Modified ReceiptQrCode to work with CAEA.

6. Modified PDF generation to allow creation of PDF with CAEA (This receipts will be unvalidated at that moment).

7. Added some tests to ensure that everything works as expected.

[WhyNotHugo]

Thanks for your patience on this one!