Merge pull request #91 from Fang-/created-at-conditional

created_at timestamp is conditionally included

docs/tech/json.md

@@ -86,7 +86,7 @@ This location object describes the location of the device that reported it.
 * `inrids` contains a list of region IDs the device is currently in (e.g. `["6da9cf","3defa7"]`). Might be empty. _(iOS,Android/list of strings/optional)_
 * `SSID`, if available, is the unique name of the WLAN. _(iOS,string/optional)_
 * `BSSID`, if available, identifies the access point. _(iOS,string/optional)_
-* `created_at` identifies the time at which the message is constructed (vs. `tst` which is the timestamp of the GPS fix) _(iOS,Android)_
+* `created_at` identifies the time at which the message is constructed (if it differs from `tst`, which is the timestamp of the GPS fix) _(iOS,Android/integer/epoch/optional)_
 * `m` identifies the monitoring mode at which the message is constructed (_significant_=`1`, _move_=`2`)  _(iOS/integer/optional)_
 * `_id` random identifier to be used by consumers to correlate & distinguish send/return messages _(Android/string)_