Sniff out GDAL data under sys.prefix

This should let eg Conda users avoid setting a GDAL_DATA env var.

I would have preferred to use SetConfigOption() but couldn't get that to work. I'll remind myself to look upstream and see what's up in GDAL.

Closes #216.
@sgillies in cases of simple edits like this, would you prefer I:
1) flag this for you to fix, then I merge when you are done      (this is closest to our process at CBI and why I defaulted to this)

OR 

2) I make edits directly in your pull request, and ping you for approval

OR 

3) I make edits then merge