Make it possible to use microtime as the version value when using Versionable

This patch basically makes it possible to pass in a custom auditlog listener as well as providing a new listener that uses microtime to generate the version value's instead of incrementing an integer value starting at 1. This should provide a much better handling for concurrent inserts and should also be faster, since for all but the first version a complex select is necessary in the current implementation.

Open question is if the accuracy multiplier should be an option or not. Also is there any reason why the column definition doesnt default to an unsigned, not that it matters all that much?