Add possibility to import vCard contacts from the SD card

The user should be able to import vcards from the memory card into the Contacts app. Vcard is the standard in contact format and most feature phones export to it. Firefox OS should be able to import these contacts easily from the Contacts app and in the FTU.

This patch looks in the SD card for any VCF file and imports it in the contact list, in a similar way of what the SIM importer does. It doesn't check for duplicates as of now.