Filter out zap frames from stack traces to reduce noise

[prashantv]

Zap currently only filters our runtime stack frames and the last takeStacktrace function, but not the other zap frames. This leads to stack traces like:

From #308 (comment)

2017-05-02T22:03:11.186-0700	ERROR	config/postgres.go:32	Unable to connect to postgres database{error 25 0  dial tcp [::1]:5432: getsockopt: connection refused} {config 2 0  postgresql://gecko_admin:devpass@localhost:%!d(string=5433)}	{"app_name": "ProjectGecko", "version": "0.1.0"}
go.uber.org/zap.Stack
	/Users/matt/Programming/GoDevel/src/go.uber.org/zap/field.go:209
go.uber.org/zap.(*Logger).check
	/Users/matt/Programming/GoDevel/src/go.uber.org/zap/logger.go:273
go.uber.org/zap.(*Logger).Check
	/Users/matt/Programming/GoDevel/src/go.uber.org/zap/logger.go:146
go.uber.org/zap.(*SugaredLogger).log
	/Users/matt/Programming/GoDevel/src/go.uber.org/zap/sugar.go:223
go.uber.org/zap.(*SugaredLogger).Error
	/Users/matt/Programming/GoDevel/src/go.uber.org/zap/sugar.go:102
gecko/config.SetupPostgres
	/Users/matt/Programming/ProjectGecko/src/api/src/gecko/config/postgres.go:32
main.main
	/Users/matt/Programming/ProjectGecko/src/api/server.go:40

Or if the package is installed through a vendor directory:

Error message
github.com/prashantv/test/vendor/go.uber.org/zap.Stack
	github.com/prashantv/test/vendor/go.uber.org/zap/field.go:191
github.com/prashantv/test/vendor/go.uber.org/zap.(*Logger).check
	github.com/prashantv/test/vendor/go.uber.org/zap/logger.go:301
github.com/prashantv/test/vendor/go.uber.org/zap.(*Logger).Check
	github.com/prashantv/test/vendor/go.uber.org/zap/logger.go:172
github.com/prashantv/test/vendor/go.uber.org/zap.(*SugaredLogger).log
	github.com/prashantv/test/vendor/go.uber.org/zap/sugar.go:233
github.com/prashantv/test/vendor/go.uber.org/zap.(*SugaredLogger).Errorf
	github.com/prashantv/test/vendor/go.uber.org/zap/sugar.go:148
github.com/prashantv/test/[...]

The Errorf etc don't really provide much value, so we should remove zap stack frames, even when they're in vendor.