added st_geomfromgeojson support.

Hi, 
  Thanks for creating this extension.
  
  I've added a function to support st_geomfromgeojson. 
  When the JSON.parse call fails, i've set it to throw a new error instance, which seems 
  to be the standard behavior in Knex functions without callbacks.

  The test passes, but let me know if you want me to modify anything.

Sebastian.