RPM::Spec: store files/excludes on a per-subpkg basis
Add various file detection utility methods to Gem and RPM::Spec
Rework gem/rpm file detection / update process

Incorporates subpkg-specific file detection just added
RPM::Spec - Invalidate local gem when metadata is updated
Refactor spec conversion mechanism into modular sub-methods

Slightly rechanges internal workflow, spec contents is now
updated in RPM::Spec#update_to and to_string method simply
returns updated spec.

Update process is rather crude atm, just manual string
parsing / splitting / interpolation, could use some
refactoring
Add / Update Specs to cover latest features