a transaction manager for nested transactions

New class: AgileWiki.ark.tks.cachefile.TransactionManager. (Now under subversion.)

This new class focuses only on handling the before image file for a transaction. Because of the narrow focus, its pretty simple. (And I didn't try anything fancy, either, like trying to cache before images.) And given a stack of TransactionManager instances, managing nested transactions shouldn't (!!!) be too difficult.

Lets see how it goes.

Bill