Stoner

Second bugfix version of stable branch

First bugfix release of the current stable branch

Fixed a bug that caused add_column to add headers twice

First release candidate for version 0.6.0 - start of a new stable branch.

Second beta release of version 0.6. This has revised exception handling for cases where the setas attribute is not defined - now a subclass of AttributeError - StonerSetasError is raised.
Various other bugs squashed.