SMW\SPARQLStore tidy some classes + renaming

- Move 'SMWSparqlDatabaseVirtuoso' to VirtuosoHttpDatabaseConnector
- Move 'SMWSparqlDatabase4Store' to FourstoreHttpDatabaseConnector
- Previous class names are kept as alias to aid transition
Merge pull request #371 from SemanticMediaWiki/sparql-tidy

SMW\SPARQLStore tidy some classes + renaming