Update ST_Extent
Update and add image ST_Extent
Latest update ST_Extent
Update ST_Extent
Corrrection of image and doc ST_Extent
Latest correction of ST_Extent
Update ST_Extent
Add missing signature and examples.
Merge pull request #2 from agouge/fix-st-extent

`ST_Extent` is an aggregate function, so you need to have at least one aggregate example. Can you add an illustration for this example?

Note the last example I added returns a `POINT`.

I also added a missing signature.
Update image and example ST_Extent
pdate link of ST_Envelope in ST_Extent
delete an image of ST_Extent
Merge branch '128' into gh-pages

Update and add image St_Extent