Add RFC3339, RFC3339Nano and update docs to include 'nano'

[uhthomas]

I was going to open an issue but #659 is already open.

Go's time package suggests that 'RFC3339 should be preferred for new protocols' so it would be great to have RFC3339 as a standard time format in zap.

[CLAassistant]

All committers have signed the CLA.

[prashantv]

Thanks for the contribution @uhthomas

Can you please include unit tests in encoder_test.go

[uhthomas]

@prashantv Absolutely! I've added some unit tests :)

[codecov]

Codecov Report

Merging #736 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #736      +/-   ##
==========================================
+ Coverage   97.43%   97.44%   +<.01%     
==========================================
  Files          40       40              
  Lines        2109     2117       +8     
==========================================
+ Hits         2055     2063       +8     
  Misses         46       46              
  Partials        8        8

Impacted Files	Coverage Δ
zapcore/encoder.go	84.61% <100%> (+1.48%)	⬆️

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[uhthomas]

Awesome, thanks for the approval @prashantv! What's required to get this merged?

[prashantv]

Nothing else needed, I'll merge now :)