Implement renumerator

[krother]

Exchange ideas on the interface after extracting code from fixfrag.

[krother]

Hi Pawel,

I've extracted my renumeration code from the fixfrag branch. I have added it to a branch but not pushed it yet. Let me know when you are ready picking the code you want to preserve.

KR

[cosi1]

Hello Kristian,

I've already cloned the repository, so now I have all the code to be preserved on my machine. It's our discussion in #40 that stopped me from tinkering with it; I badly need to think about a good interface first and then start coding. (The other thing that stopped me is two large and urgent projects on my head ;-)).

As soon as I send you the new version of Renumerator.py, I think that we can remove the whole fixfrag branch and move back to the old version where all the tests passed.

[lenarother]

Some comments in #30.