support for ecdsa ssh key.

- add ssh key support (ecdsa)
- update validator.py
- modify help.py balloon and add warning

Regards!
It seems to me that the regular expression is doing more than just checking if ``keyline`` is in the list ``['rsa', 'ssh-rsa', 'ecdsa', 'ecdsa-sha2']``.

Have you tried something like:
```
validline = re.match('^(%s) [ \t]*[^ \t]+.*$' % '|'.join(self.valid_ssh_key), keyline)
```

No sure it will do what you want to it seems closer to me to the original idea.