Core method for Transfer

I would like Adhearsion to have a `#transfer` CallController method.  For simplicity, let's start with a blind transfer.

The effect should be something like:

```Ruby
def transfer(destination)
  write_and_await_response Punchblock::Command::Redirect.new(to: destination)
end
```

The only problem with this method is that the calling CallController attempts to continue.  The effect of transfer should be understood to implicitly end the call like a `#hangup`.