Happy to change the prefix etc...

Also make minor docstring amendments.

Also made some notes regarding Downing.

I believe the logic was slightly faulty for Tullock: the first 11 moves
was correct however once past there we should only be considering the 10
previous moves.

As noted by @id428 we were not giving a "true" fresh start so I've
implemented that. I also added the two final defections (when the game
length is known).

This is a complete rewrite of the Downing strategy.

To be able to do this I've used the description in Downing's 1975 paper.
This description itself is not sufficiently clear and so I've had to
make some further assumptions which I've clearly documented.

Note: there was documentation claiming that there was a bug in the
implementation in the original tournament. I believe this was a mistake
due to a misinterpretation of one online set of slides where they
commented that there was a mistake in the implementation. This however
was not a bug and was actually described quite a lot in Axelrod's
original tournament: the strategy was implemented to act a particular
way in the first two rounds and this had the result of making the
strategy a king maker. This however was not a bug, just a particular
interpretation of the overall decision rule described in Downing's 1975
paper.

Note that this was not the specific error that @id428 pointed out but
having reviewed the papers and source code I found this one minor
inaccuracy.

@id428 made a point that the strategy should cooperate twice after it's
round of retaliations but I do
not see this in any of the descriptions of the strategy. Once the
strategy has finished retaliating, all the texts indicate that it
cooperates again but ready to retaliate.

I have moved this to it's own module/file.