Configurable message synchronisation since date provided by the end-user

[tuxpiper]

The datasource should accept a "Get messages since ..." date , so that the user can configure from which point in time it wants to retrieve messages from the inbox.

Possible values:

• null : no date, meaning that only new messages from the moment of activation should be provided. This should be the default option selected in the user interface.

• date between UNIX epoch and today (no future dates allowed): means that messages should be first retrieved starting from the given date. [ Let's make sure to get the timezones idea right ]

Aha! Link: https://ushahiditeam.aha.io/features/PROD-100