Add a log adapter for zap.Logger

[MaruHyl]

For #854

[MaruHyl]

@peterbourgon done.

[ChrisHines]

It would be nice to have some test cases that probe the edge cases of the varargs kv passed to Log. Otherwise I made a few comments about readability.

[MaruHyl]

@ChrisHines Hi, I rewrote the test according to your suggestion.

[ChrisHines]

One more improvement then we're good.

[MaruHyl]

@ChrisHines ok, use subtests now, test output

=== RUN   TestZapSugarLogger
--- PASS: TestZapSugarLogger (0.00s)
=== RUN   TestZapSugarLogger/debug
    --- PASS: TestZapSugarLogger/debug (0.00s)
        zap_sugar_logger_test.go:83: {"L":"DEBUG","T":"2019-04-09T12:26:09.090+0800","M":"","key1":"value1"}
=== RUN   TestZapSugarLogger/info
    --- PASS: TestZapSugarLogger/info (0.00s)
        zap_sugar_logger_test.go:83: {"L":"INFO","T":"2019-04-09T12:26:09.091+0800","M":"","key2":"value2"}
=== RUN   TestZapSugarLogger/warn
    --- PASS: TestZapSugarLogger/warn (0.00s)
        zap_sugar_logger_test.go:83: {"L":"WARN","T":"2019-04-09T12:26:09.091+0800","M":"","key3":"value3"}
=== RUN   TestZapSugarLogger/error
    --- PASS: TestZapSugarLogger/error (0.00s)
        zap_sugar_logger_test.go:83: {"L":"ERROR","T":"2019-04-09T12:26:09.091+0800","M":"","key4":"value4"}
=== RUN   TestZapSugarLogger/dpanic
    --- PASS: TestZapSugarLogger/dpanic (0.00s)
        zap_sugar_logger_test.go:83: {"L":"DPANIC","T":"2019-04-09T12:26:09.091+0800","M":"","key5":"value5"}
=== RUN   TestZapSugarLogger/panic
    --- PASS: TestZapSugarLogger/panic (0.00s)
        zap_sugar_logger_test.go:83: {"L":"PANIC","T":"2019-04-09T12:26:09.091+0800","M":"","key6":"value6"}
PASS

[ChrisHines]

@MaruHyl Thanks for the contribution!