Removes the actual key from Keychain

This is a proposed fix for #21.
@segiddins If you `$stderr.puts delete_generic`, you receive something like this, if it's a success:

```
password has been deleted.
keychain: "/Users/user/Library/Keychains/login.keychain"
class: "genp"
attributes:
    0x00000007 <blob>="cocoapods-keys-Project"
    0x00000008 <blob>=<NULL>
    "acct"<blob>="asdf"
    "cdat"<timedate>=0x3230313431320363030343435395A00  "20141206004459Z\000"
    "crtr"<uint32>="aapl"
    "cusi"<sint32>=<NULL>
    "desc"<blob>=<NULL>
    "gena"<blob>=<NULL>
    "icmt"<blob>=<NULL>
    "invi"<sint32>=<NULL>
    "mdat"<timedate>=0x3230313413230363030343531335A00  "20141206004513Z\000"
    "nega"<sint32>=<NULL>
    "prot"<blob>=<NULL>
    "scrp"<sint32>=<NULL>
    "svce"<blob>="cocoapods-keys-Project"
    "type"<uint32>=<NULL>
```

However, if it fails, you get

```
security: SecKeychainSearchCopyNext: The specified item could not be found in the keychain.
```

So I don't think it would be best to switch them.