Add git submodule support

[bergie]

If a installed component contains git submodules (i.e. .gitmodules is set), then those modules should also be installed and updated. This means running:

• git submodule init
• git submodule update

[bergie]

This basically affects https://github.com/midgardproject/midgardmvc_installer/blob/master/lib/midgardmvc_installer/pakeMidgardMvcComponent.class.php#L190 and https://github.com/midgardproject/midgardmvc_installer/blob/master/lib/midgardmvc_installer/pakeMidgardMvcComponent.class.php#L240

pakeGit needs the necessary methods for dealing with submodules: https://github.com/indeyets/pake/blob/master/lib/pake/pakeGit.class.php