Improvements in wbs_core

The following functionality was added to the wbs_core module:

Fixed an error when trying to copy properties to an empty wikibase instance. Now when the listProperties method of the WikibaseEngine is called, an empty list is returned when the target wikibase is empty:
```
def listProperties(self):
if 'query' not in properties:
    # wikibase is empty
    return []
...
```
Now the listProperties method returns the complete list of properties of the wikibase using the 'continue' param when needed, instead of just returning the last 10. This fixes an error when sometimes a property that already exists in the target wikibase was trying to be created again.
The listProperties method is just called once when creating a new property, improving the performance when copying properties.
Added tests for some of the methods of the module.
Added docstrings for the copyProperties and listProperties methods.

SuLab / WikidataIntegrator