Allow for excluding specific files when creating an archive

In some cases it's necessary to exclude specific files when archiving
directories recursively, e.g. when their filename is broken. This is now
possible by giving an array of paths to exclude to System#create_archive
Also exclude files with broken filenames from the tarball

Until now problematic files were excluded from the metadata, but were
still put into the archive when the file was part of an unmanaged tree.

Fixes #94
Merge pull request #98 from SUSE/review_140815_do_not_extract_broken_files

[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_140815_do_not_extract_broken_files'