support generic API for a online backup wallet storage service

[shackra]

besides the local backup, people would like to backup theirs wallets online. I request a generic API in bitcoin-qt for online backups. It should be generic because it haven't favor any particular existing or future backup online service and any interested to offer the service should build his API around the bitcoin-qt's online backup API specification.

this should be just the URL plus the API's root (i.e: someonlinebackupservice.com/api/backup) and the space to specified an API Key by the user, this should go to the Options window though.