MEP-5 ISO timestamps

based on #460 
Yes it does. However, before, it would have crashed as a couple of lines later when using ToLong() [here](https://github.com/Open-Transactions/opentxs/pull/509/files#diff-c3bbfb2ef578250e7d70ed7d5300f266L229).

This PR deals with conversion to the new format only, and does not contain any other changes.