Support the option to `get` by SSH of GitHub

It gets by SSH of GitHub if run `ghq get` with `-p` option
and arguments isn't absolute URL (means omitted).

This option has the compatibility with hub(1).
Support the option to import with starred by SSH of GitHub
Add documentations for command line option to get SSH
Integrate duplicated logic to construct SSH URL
Merge pull request #7 from moznion/option_to_get_by_ssh

Support the option to `get` and `import` by SSH with GitHub