WKT Proj4 Extension support

[pomadchin]

WKT strings can be of the following shape:

PROJCS[
  "WGS 84 / Pseudo-Mercator",
  GEOGCS["WGS 84",
    DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]], 
    UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]
  ],
  PROJECTION["Mercator_1SP"],
  PARAMETER["central_meridian",0],
  PARAMETER["scale_factor",1],
  PARAMETER["false_easting",0],
  PARAMETER["false_northing",0],
  UNIT["metre",1,AUTHORITY["EPSG","9001"]],
  AXIS["Easting",EAST],
  AXIS["Northing",NORTH],
  EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null +wktext +no_defs"],
  AUTHORITY["EPSG","3857"]
]

Right now parsing of such a string is not supported (parser will throw an exception, trying to parse EXTENSION).

The task is to add into parser EXTENSIONS support and to use PROJ4 Extension to convert WKT string into CRS by parsing the proj4 string from the Extentions field.