Discovery command for github private repositories

Added discovery command which can be used for satis.json file generation through different API's (currently there is only Github implementation).

Implementation covered by phpspec tests. 

Sample usage:

```
/bin/satis discover some-organisation
```

More advanced usage:

```
/bin/satis discover --name='My repo name' --homepage="http://example.com" -e some-organisation/some-repo some-organisation other-organisation > ./satis.json
```

Discovery command will use composer configuration for authentication but you may also provide github token via command line option. 
I would suggest using the ``->organization()`` API instead, which allows to have type information for the return type, making this much more usable in IDEs (or in static analysis tools)