Creation of EntityProvider to get entities

The functions WikibaseApi::getEntityFromId and WikibaseApi::getEntityFromSitelink
are deprecated and will be removed.

The functions WikibaseApi::getEntitiesFromIds and WikibaseApi::getEntitiesFromSitelinks
will, in the futur, return the API result and not an Entity array.

This will split the library in two parts:
- interaction with the API using WikibaseApi class
- manipulation of Wikibase content using helper classes (Entity, Claim...)