Added pubkey authentication

Added public key authentication with support for password protected private keys (see syncany/syncany#134).

If a user doesn't provide a private key during the initialisation password-based authentication will be used. Password field is used to provide either the private key's password or a user's ssh password.

Altered setup:
```
Connection details for SFTP connection:
- Hostname: 10.211.55.6
- Privatekey Path (optional, default is none): /home/user/.ssh/id_rsa
- Username: user
- Password (not displayed): 
- Path: sy
- Port (optional, default is 22): 
```