Update schema

+Added URL column to trails.
+Tentatively added schema for POI table

I compared the coordinates data between what was in the table and original KML document. It turns out that python's float() conversion drops the 15th decimal place. I did some research and this decimal place corresponds to a level of precision higher than a micron and is probably not accurate in the first place. Because smartphone GPS hardware is accurate only to a number of meters, I believe that we only need up to the 5th decimal place (accurate to 1 meter).