Relax UTCTime format string

It's quite common to encounter timestamp strings of format:

2014-04-28 13:13:13.112233
2014-04-28 13:13:13

(I.e., without 'T' in the middle and without the 'Z' postfix)

An example source of these strings is SQLite:

sqlite> select datetime('now');
2014-04-28 12:35:12

Rather than fail to parse these strings, allow them in JSON -> UTCTime
conversion.

See #197 for more examples & test case that reproduces the failure.