Sort candidates by previous completion choices

Here is an experiment I am working on: Store how often candidates are selected and use this to rank the offered candidates with an entry in company-transformers.  The most complicated thing so far was resizing the completion history ring (probably hardly ever used in practice :).

I plan on writing some tests, and on extending it to distinguish b/w files or projects, but I am not really sure what I want for that.  Anyway, there are probably lots of bugs and inconsistencies that I'd be happy to hear about, besides any other comments you might have!