default category name: GENERAL -> last part of caller's package name

e.g. if the calling package is Foo::Bar::Baz, the default category is "Baz", rather than "GENERAL".

When it comes to searching, categorising a message as "GENERAL" is the same as not categorising it. This patch makes Log::Declare default to a (short) version of the calling package name, which is what I assumed it was already doing :-)