Add a `convertRawValue` method to the `Specification` class.

The `convertRawValue` method can be used to convert a VBus field
value from one unit to another as long as the two units are in the
same unit family.
Add `timeToLive` option to `Recorder` class.

The value of `timeToLive` is forwarded to the `HeaderSetConsolidator`.
Remove a debug output from the `Recorder` class.
Fix `finish` method on `Converter` class.

The `finish` method relied on the `end` event to fire. But Node.js
does not emit this event if the stream was never read from. Adding
a `data` event handler caused the stream to at least read once and
hence emit the `end` event correctly.
Refactor some debug output in `FileSystemRecorder` class.
Fix some `finish` / `end` mix-ups.