Implementing both feature #81 and #61

Implemented #81 and #61 
`default_datastore` wouldn't be ok, as datastore is related with resouces inside a package/dataset

Regarding `default_dataset` vs. `default_package` both terms are used in CKAN. Having a look to the documentation API at http://docs.ckan.org/en/ckan-2.2/api.html#the-ckan-api it seems that dataset is more extended (i.e. known by potential users) so I'd suggest to keep with it.