Add swiss bank statement imports

New module that adds several import types to the module 'account_statement_base_import', in order to read swiss bank statements file.

It currently supports the following file formats :

.v11, .esr, .bvr formats (ESR standard) for records of type 3 (type 4 is ready to be implemented)
.g11 format from Postfinance S.A. for Direct Debit records of type 2
XML format from Postfinance S.A.
.csv format from Raiffeisen Bank
.csv format from UBS Bank [CH-FR]
use relative imports (`from . import xxxx`)