Plot enhancement

The current implementation of `Logger.lua` doesn't consider the order of the different `keys`.
It is a problem if you want to use the file afterward, and also the legend is shuffled.

My implementation allows the user to keep the intended order, but it is also compatible with the previous usage.

See below the result of the `test/test_logger.lua`:

![current](https://cloud.githubusercontent.com/assets/4975335/3150047/365bf49c-ea71-11e3-82e7-7cf692326b1f.png)

![new](https://cloud.githubusercontent.com/assets/4975335/3150051/3ace7a4a-ea71-11e3-845f-a23b131f58ec.png)