Add rfc3339 timestamps to default format

[KodrAus]

Closes #30

Adds RFC3339 formatted timestamps to the default log format:

I've made an effort to avoid allocating temporary strings or needing to parse a format for each log, so there's a currently private Timestamp struct that can be pulled off a Formatter that does this.

[KodrAus]

r? @mjkillough

[mjkillough]

@KodrAus - Apologies for the late review, I was on holiday until this afternoon.

This looks great! :) I didn't realise how great the formatting in chrono was.

[tailhook]

Well, since the UTC time is used anyway (which is a good thing), I think +00:00 is excessive, Z is usually used in that case and is clear enough and shorter (so easier to observe in console, since this is the primary target of env_logger). Also, I think that nanosecond precision, while can be occasionally useful is also excessive. I'm usually okay with second granularity. May be millisecond if possible.

In my opinion time::now_utc().rfc3339() is good enough and produces value that looks like: 2017-01-01T00:00:00Z, and also avoids few more dependencies.

[KodrAus]

Thanks @mjkillough and @tailhook!

The time crate is effectively deprecated in favour of chrono so I don't think we should depend on it directly. I think the points about the format are good, we might end up having to reconstruct a simpler rfc3339 format manually though. But that's not the end of the world.

[KodrAus]

Added #34 to sort the format out.