Add support for using env variables instead of .ftppass

This commit adds support for declaring the authentication parameters as environement variables instead of inside the .ftppass file.
Passing the variable name in the grunt task's `authKey` parameter or the `.ftppass` configuration is seamless.

Example:

**~/.bashrc** (or equivalent)
```bash
export GRUNT_SFTP="{
                      \"username\": \"your-username\",
                      \"keyLocation\": \"/home/louis/.ssh/izi_rsa\"
                   }"
```

**Gruntfile.js**
```javascript
"sftp-deploy": {
    build: {
       auth: {
           host: 'my-ftp.com',
           port: '5678',
           authKey: 'GRUNT_SFTP'
       }
      ...
    }
}
```