Gdal like tiling scheme tags

[vincentsarago]

closes #192

This PR does

• [x] Use same namespaced metadata for Tiling Scheme as the GDAL COG driver
• [x] Update the Info Model to make Tags a Dict of dict

rio cogeo info web.tif --json | jq .Tags
{
  "Image Metadata": {
    "AREA_OR_POINT": "Area",
    "DataType": "Generic",
    "OVR_RESAMPLING_ALG": "NEAREST"
  },
  "Image Structure": {
    "COMPRESSION": "DEFLATE",
    "INTERLEAVE": "BAND",
    "LAYOUT": "COG"
  },
  "Tiling Scheme": {
    "NAME": "WEBMERCATORQUAD",
    "ZOOM_LEVEL": "17"
  }
}