Improve separation of files between main and doc packages

I bring three more improvements for updating spec files:

#### Include only runtime files in the main package

Only executable files (code) alongside the licensing should be in the main package since we have '-doc' subpackage. I believe it's easier and more convenient to actually list those files in the main package and put everything else that we didn't catch into '-doc'.

What should be caught?
- lib/ - this is true for every gem
- bin/ - some gems have bin executables
- app/ - some gems come with code that extends your Rails application
- vendor/ - this is not so common, but if present, also contains necessary files

Does anything else cross your mind?

#### Improve guessing %doc files

Allow both upper and lower case and add History, Release and doc/ files if present. History and Release files usually contain changelog entries.

#### Turn /bin in spec to both %{_bindir}/* and %{gem_instdir}/bin/*

Turning /bin/* files to %{_bindir}/* is not enough. Both %{_bindir}/* and %{gem_instdir}/bin/* should be stated.