Better RPM spec updates

Hi,

as a Ruby packages maintainer I really welcome any automation effort and polisher is already so far that I would really like to use it for my semi-automatic spec updates. However, to be able to do this I have to bring polisher and associated update script to the point it matches the Ruby guidelines and works for most of our gems in Fedora. Today I started this effort and I bring the first parts of improvements.

Namely:

1, include license files in the main package (according to the guidelines and most gems already in Fedora)
2, increase the list of possible license files according to what upstream projects use
3, properly mark files as %doc (this is important especially for license files that should be in the main package)
4, prefer packager name coming from `rpmdev-packager` as this is what RPM packagers likely use (they probably use `rpmdev-bumpspec` which use the same source) 
5, fix update message by including the gem's name
6, add in-place update option to the spec update script so it can be used very easily

Thank you for the consideration, it's a only a start, but I thing the stuff coming in this PR is really essential and common (nothing much to argue about I hope).

Yes, once we agree on this stuff here, I plan to do changes to the gem2rpm defaults as well.