Adding File object(s) support to file transfer methods

This pull request adds support of a File object or its collection to the get method and the put method. Below examples will work.

```groovy
task deploy(type: SshTask, dependsOn: war) {
  session(remotes.webServer) {
    put war.archivePath, '/webapps'
  }
}
```

```groovy
task deploy(type: SshTask, dependsOn: war) {
  session(remotes.webServer) {
    put files('xxx', 'yyy'), '/webapps'
  }
}
```

See also #90.