[WIP] Ask confirmation for running untrusted resources

Here is a proposal to the issue #32.
The objective, is to avoid the execution of untrusted scripts

* [ ] Add tests
* [ ] Document it

```bash
$ ./bin/melody run ~/test.php  -vvv
Hello Melody!

$ ./bin/melody run 23bb3980daf65154c3d4 
You are running an untrusted resource
  URL: https://gist.github.com/23bb3980daf65154c3d4
  Owner: lyrixx
  Created at: Fri, 17 Oct 2014 14:44:13 +0000
  Last update: Fri, 17 Oct 2014 23:45:48 +0000
  Revision: #2
  Trust signature: 9d9f9ba5a46b75d1117709a621f5860ea46c6b1979c176cddad072ccde0a1482
Do you want to continue [y/N]?y
----------------------------------
Hello greg
----------------------------------

$ ./bin/melody run 23bb3980daf65154c3d4 --trust-all
----------------------------------
Hello greg
----------------------------------

$ ./bin/melody run 23bb3980daf65154c3d4 --trust-only 9d9f9ba5a46b75d1117709a621f5860ea46c6b1979c176cddad072ccde0a1482
----------------------------------
Hello greg
----------------------------------
```
I would write:

```php
$this->metadata = $metadata ?: new Metadata()
```