_creation_date and _last_modified have inconsistent implementations

[lwcolton]

_creation_date and _last_modified will have different values depending on which plugin you are using.

In peartree:

json_payload[label]['_last_modified'] = datetime.now().isoformat().split(".")[0]

In botostore:

params['_creation_time'] = str(datetime.utcnow())

Two problems:

• We are using datetimes and not epoch timestamps
• We are applying timezone offsets in some plugins but not others

Proposed solution:
Add _created and _modified which will both be epoch timestamps. Existing plugins may continue to implement _last_modified and _creation_time for backwards compatibility, but they will be deprecated and phased out.

Please advise if this is acceptable.