protect import of external dependencies

Odoo won't install an addon if the external dependencies are not met.
However, the python modules of the addons are imported at startup, and the
lack of an external dependency for an external addon will cause a crash,
therefore the import needs to be in a try..except block.

account_banking_sepa_credit_transfer/models/account_banking_sepa.py

@@ -22,7 +22,11 @@
 
 from openerp import models, fields, api
 from openerp.addons.decimal_precision import decimal_precision as dp
-from unidecode import unidecode
+
+try:
+    from unidecode import unidecode
+except ImportError:
+    unidecode = None
 
 
 class BankingExportSepa(models.Model):