Bootstrap for PDO API implementation

[JanJakes]

Extracting smaller chunks from #291 so we can review and merge them independently.

This PR renames the current WP_SQLite_Driver to WP_PDO_MySQL_On_SQLite and reintroduces the WP_SQLite_Driver as a simple proxy over the new renamed class. It's done in these two steps (first rename, then reintroduce) so that Git understands the rename and presents history (hopefully) accurately.

The changes are better understood in a commit-by-commit view.

WP_PDO_MySQL_On_SQLite vs WP_SQLite_Driver

The "reintroduced" WP_SQLite_Driver is not meant to be permanent. It is a temporary proxy so we can gradually modify WP_PDO_MySQL_On_SQLite to support PDO APIs while not touching the driver API just yet. Once the basics of PDO API are in place, we can make all dependencies use the new class directly and then remove the WP_SQLite_Driver.

That is, in the future, the WP_SQLite_DB extends wpdb will use directly the new WP_PDO_MySQL_On_SQLite class.

[adamziel]

reviewing this commit by commit it seems like a fairly small PR despite what the files changed tab says. Looking good!