"as" methods for SpatialPoints and SpatialLines.

sp packaged does not have methods for as.SpatialPointsDataFrame.SpatialPoints or as.SpatialLinesDataFrame.SpatialLines (only as.SpatialPolygonsDataFrame.SpatialPolygons). This implements it for Lines and Points
geojson_write methods for SpatialLines and Points
Merge pull request #14 from ateucher/reworkapi

Adding geojson_write methods for SpatialLines, SpatialLinesDataFrame and SpatialPoints