RFE: sign RPMs and yum repositories

Merfi can only sign Debian package repositories. We should make it search for RPM repositories and sign those, too.
## Sign RPMs

Currently our shell script searches the tree for any file named `.rpm`, and checks for an existing signature, like `rpm -qi -p $rpm 2>/dev/null | grep ^Signature`

If that signature matches our key ID, then we assume the RPM has already been signed, and we skip it and move to the next one. If the Signature field is not present, or it does not match our key ID, then we sign the RPM (and move on to the next one, repeating the process).

Lastly, we need to re-run `createrepo` after we do this, because we're altering the RPM files' checksums whenever we change the signatures.
## Sign yum's `repomd.xml` as `repomd.xml.asc`:

This is relatively straightforward. After we've completed the above steps of `rpm --addsign *` and `createrepo`, just detach-sign the `repomd.xml` file:

```
gpg --batch --yes --detach-sign --armor -u 0123ASDF repomd.xml
```