Add supports for gem archives

Gem archives are tarballs containing three elements:
- metadata.gz
- data.tar.gz
- chechsums.yaml.gz

We are only interested in the data.tar.gz

Fixes https://github.com/fedora-infra/summershum/issues/6
Use os.path.join to join multiple element in one single path
Check that the filename retrieved points to a directory

Otherwise it means the content of the archives had no subfolders and thus we should browse the full tmpdir
Remove the archive before browsing the extracted files

This prevent us indexing the archive itself
Remove FIXME now that we fixed it
Fix figuring out the folder name for zip archive

This fixes digesting the files of maven-gpg-plugin-1.5-source-release.zip
package: maven-gpg-plugin