`crs` assignment after `read..` or creating methods for GeoDataFrame

[AndreiKingsley]

1. Add optional crs parameter to read.. methods for GeoDataFrame
   Need some research here, is it valid or not in general.
   The Shapefile always contains explicit CRS field, so no additional parameter is needed in readShapefile()
   The GeoJSON Specification claims that CRS always should be WGS84.
2. Add crs filed in GeoDataFrame not nullable.