injectedlogger: fix typo in comments
injectedlogger: rename `declare` to `use`

While at it also call signature, so that it received required log
levels as normal parameters.

Also change comments and expand examples.
injectedlogger: replace if not with unless for easier reading
injectedlogger: forward InjectedLogger.inject{,!,ed?} to InjectedLogger::Logger
injectedlogger: use forwarders for inject and injected?
logger: add add_levels method
injectedlogger: make sure we try to create new log levels when required and not present
README: adapt examples a bit to new methods
injectedlogger: release 0.0.7
misc: some quick tests while we dont have specs