Improve error handling:
* Geometry.ToWKT: underlying C function always returns OGRERR_NONE, so return err == nil in this case.
* Layer.NextFeature: change to use "comma ok" return style, returning ok == false when the underlying feature iterator has reached the end of the layer
* OpenDataSource: change to add error return value, returning non-nil error if open fails
* DataSource.Release: return nil on successful release

Miscellaneous changes:
* Geometry.ToWKT: free WKT string allocated by C function, per the GDAL API docs
* Fix typo: Feature.UnnsetField
* Fix typo: Feature.StlyeString
* Add some documentation about the need to release/destroy objects returned