Support for deleting all keychain items matching a query.

I think that -deleteItem: method of SSKeychainQuery is way too restrictive in its precondition checks. According to its description in the header file it is supposed to delete all items matching a query (and TBH this is what I would've expected as well), but in reality it only allows deleting items with specified service and account.
I also feel that method names in SSKeychainQuery are a bit incoherent with each other, but that's probably very subjective.
This pull request is the way I would've done it, but I guess that for the sake of retaining backwards compatibility a simple addition of a -deleteAll: method or the like will suffice.