Detect missing tables and create them if required

[bonny]

Reported in support forum: https://wordpress.org/support/topic/missing-table-support/.

And probably by others too because I recall that other users have had similar problems.

Probably this happens when information about Simple History is in the options table, but the actual Simple History tables are missing.

[bonny]

Next version, 4.5, will have a check in the debug page to detect missing tables. It will not yet create them, but it will simplify debugging nonetheless.