Add RFC3339NanoFormatter and UnixNanoFormatter

[errnoh]

Adds support for two different methods of logging in nanosecond precision without losing information.

Both are new TimeFormatters, so that no existing code will break. RFC3339NanoFormatter is similar to RFC3339Formatter but uses time.RFC3339Nano instead of time.RFC3339, and UnixNanoFormatter uses time.Time.UnixNano() int64 output that is fast to both serialize and read back in time.Time format.

See #275 for the issue in question.

• signed Uber's Contributor License Agreement;
• added tests to cover your changes;
• run the test suite locally (make test); and finally,
• run the linters locally (make lint).

[mention-bot]

@errnoh, thanks for your PR! By analyzing the history of the files in this pull request, we identified @akshayjshah to be a potential reviewer.

[CLAassistant]

All committers have signed the CLA.

[akshayjshah]

LGTM. Thanks for the contribution, @errnoh! Keep in mind that we're getting close to releasing a stable 1.0 of zap, and many of these APIs will change. I'll make sure to keep a handful of nanosecond-precision options available with the new APIs, too.