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.
Consider a little consolidation:

```
gdal_root = os.path.join(sys.prefix, 'share/gdal')
if os.path.exists(os.path.join(gdal_root, 'pcs.csv')):
    os.environ['GDAL_DATA'] = gdal_root
```