Ask confirmation for running untrusted resources

Here is a proposal to the issue #32.
The objective, is to avoid the execution of untrusted scripts

* [x] Add tests
* [x] Document it
* [x] Remember trusted signatures
* [x] Trust users

```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

Do you want to continue [y/N]?y
----------------------------------
Hello greg
----------------------------------

$ ./bin/melody run 23bb3980daf65154c3d4 --trust
----------------------------------
Hello greg
----------------------------------
```
I would return a `UserConfiguration` or null