Add 900913 EPSG code to cubewerks_extra file

[hobu]

PROJCS["Google Global Mercator",
    GEOGCS["Sphere",
        DATUM["unknown",
            SPHEROID["Sphere",6378137,0]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Mercator_2SP"],
    PARAMETER["standard_parallel_1",0],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",0],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["Meter",1],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"]]