phlsys_timedeltatostr: more helpers

Add helpers for returning string representations of durations as days,
weeks, months etc.

Include usage examples as doctests.

Test Plan:
$ ./precommit.sh
Arcyon query: more time / date formats in results

Add these new formats to the results objects:
    humanTimeSinceDate
    daysSinceDate
    weeksSinceDate
    monthsSinceDate
    humanDateModified
    humanDateCreated

Update examples/arcyon to take advantage of the new data.

Test Plan:
$ ./precommit.sh
Various in examples/arcyon