Added simplecov coverage support (default is no coverage). Also added rubocop linter with sane settings.
initial linting - a few issues remain
minor linting
added gom_path validation method
Added validation of the path when destroying.

From time to time it can happen that destroy(!) gets called with nil or empty string. This leads to deletion of '/' which normally is not as intended. We now demand that '/' has to be explicitely stated if the root node should be deleted.

destroy! nil
or
destroy ''

normally is a programming error with potentially catastrophic consequences.