Add offset (zero-based) timestamp array, refactor time conversion.

"Integer" time is now more correctly called "unix" time.
Add capability to retrieve any metric as a sequence of values.

Use an Xpath-like way of extracting single metrics as a series of
values.

Lots of opportunity for refactoring.  Maybe make a 'metric' data type
that can be subclassed to handle the various JSON object structures.

A way to translate device names would be good, esp for disk devices.