Misc fixes

What's inside of this pull request:
  * Compatibility fixes:  the first commits address some minor issues faced when running `enhancerepo` on openSUSE 13.1 (hence ruby 2.0 and latest version of all the required gems).
  * Small bugs: I found some small issues while using the tool. All the bugs have been fixed with a dedicated commit.
  * Allow the selection of the digest algorithm: the last commit makes possible to select the digest algorithm to use.

I recognize this pull request has too many things inside of it. If you want I can close it and file 3 different pull requests, each one depending on the other.