Decouple wfGetDB from Store (SMWSQLStore3Readers)

- Add setter/getter for Database
- Add SMW\MediaWiki\Database to handle MW specific tasks

SMW\MediaWiki\Database only contains methods used for the SMWSQLStore3Readers
replacement.

This will at least ensure proper tablename prefixing and enables to implement
individual tests.

## Reference
#99, #101
Merge pull request #103 from SemanticMediaWiki/store-db

Decouple wfGetDB from Store (SMWSQLStore3Readers)