Automatically converting to millisecond timestamp if the value is large enough

Change-Id: I558f11db2e662fb35428a3a561e544dd0b13989e
Merge remote-tracking branch 'origin' into wikimedia
Explicitly configuring unix timestamp units in JSONStringMessageDecoder

'unix_seconds' will convert the timestamps to milliseconds, and
'unix_milliseconds' will use the value as a long as is.

Change-Id: I76131767284a675775836cfeb18b2a06db68f11f
Merge pull request #66 from wikimedia/wikimedia

Automatically converting to millisecond timestamp if the value is large ...