elevation demo page style improvement

[hanbyul-here]

• arranged dom not to overflow, so that it can show attributions, be responsive on mobile.
• showing graph on default status, so user can notice that it is going to show some data on it.
• applied mapzen's common style button to button input.
• replacing icon to leaflet circle, so its style can be flexible.
• changed color of circle on the map to black color, so that it can pop up on colorful elevation map, synced graph color to black.