Merge pull request #1 from lento/master

Basic log package
make flag name customizable

users of the log package can choose a custom name for the flag
that matches their application needs.
simplify int/string level names, make Logger public.

Users of the package can now create their own loggers and
extend them.
remove convenience function to create flag.

We don't really need this, applications using the package can
easily create their own flag.
make Logger more customizable.

- a Logger can now write to any io.Writer.
- the error return by SetLevel indicates the valued that caused it.
gofmt