Accept various formats for last_seen/last_timestamp attribute
Enhance "last seen" attribute processing to allow a datetime string format. Basically, these attributes (last_seen or last_timestamp) can be any one of the following:
- a naive Python datetime object which is assumed to be in HA's configured time zone
- an aware Python datetime object in any time zone
- anything homeassistant.util.dt.utc_from_timestamp will accept (after being converted to a float)
- anything homeassistant.util.dt.parse_datetime will accept