Use a more granular timestamp format

https://tools.ietf.org/html/rfc5424#section-6.2.3  describes many valid formats.

heroku api logs look like this, and are valid

`2015-03-26T05:14:39.964972+00:00 heroku[api]`

logplexc lines look like this, and are valid. But 1-second granularity for app logs is not sufficient imo.
`2015-03-26T05:17:14Z app[web.1]`

fix this: https://github.com/logplex/logplexc/blob/master/minimal.go#L104
