Re-enable Attribute fixture loader

Takes the (excellent) work by @steverice and brings it into line with current EcomDev/PHPUnit conventions. Doesn't currently perform reindexing which it probably should. Refer to https://github.com/EcomDev/EcomDev_PHPUnit/pull/53 to see the necessary YAML structure (without the inline PHP). Works a treat with the Configurable fixture loader. ping @lukenm