added PlanetDetailActivity
refine PlanetDetailView
added mercury detailed text, but detail view cannot scroll yet
planet_detail view now scrollable
link message with planet_name instead of id
can get info via Planet class
now can show the picture of mars
added various planet pictures and they can be shown at detailView
made customized feed can display various images
added Earth, deleted one line of comment
added planet_detail
distance is now useless, since for some planet, it is not certain
make planet name float right to its image