Use *log.Logger instead of io.Writer for logging

[KernelPryanic]

Use *log.Logger instead of io.Writer for logging, it allows the library user to set custom logging flags and prefixes. Currently this is not possible and the default prefixes are used like: WARNING: and ERROR: , though we'd like to differentiate between error logs produced by webwire and our own code.

[romshark]

As of commit #a12e9d6 *log.Logger are now passed as log targets in both the client and the server options. When no loggers are passed then default loggers to stdout/stderr are created.