New encoder to report errors to GCP errors reporting

New log encoder to report errors through stackdriver (#42)

* new encoder,  zapdriver config

* Update logger.go

* stacktrace into message only on ErrorLevel

* inherit from zapcore.Encoder

* alter error with >= zapcore.ErrorLevel

* if stacktrace is present put it in the message

* test of grpc interceptor report on error

* Revert "test of grpc interceptor report on error"

This reverts commit 5accb62d72bd676c1832f418a9882c7b95781ae5.

* implement clone function