Prevent double-wrapping in zbark (#17)

As users and packages start using these wrappers, we're likely to start
double- and triple-wrapping loggers. Without special care, this will get
quite inefficient.

This change alters the `Zapify` and `Barkify` functions to detect
double-wrapping and recover the original logger, rather than adding
another layer of indirection.

Merge pull request #15 from uber-common/ajs-nop

Add constructor for a no-op logger

Merge pull request #14 from uber-common/ajs-zap

Introduce a bark-to-zap compatibility wrapper

Merge pull request #10 from uber-common/ajs-fix-logrus

Fix logrus import path