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.