Convert knife specs to let bindings
Add specific error messaging for SSL errors to knife

Knife now gives an error message like this when an SSL error occurs:

```
ERROR: Could not establish a secure connection to the server.
Use `knife ssl check` to troubleshoot your SSL configuration.
If your Chef Server uses a self-signed certificate, you can use
`knife ssl fetch` to make knife trust the server's certificates.

Original Exception: OpenSSL::SSL::SSLError: SSL_connect returned=1
errno=0 state=SSLv3 read server certificate B: certificate verify failed
```
Merge branch 'knife-ssl-error-messaging'