Adds a way to increase or decrease the gap between windows

Hello there, 
I made a few changes to be able to increase/decrease the gaps between windows, a feature I liked in bspwm or dwm, can't remember which.
Although I am not sure that directly changing ```config.gap``` is a good way of achieving this...
It still needs to be tested on a multi monitor setup but this should work fine as far as I understand.

Here is a small demo :

![pytyle-gap2](https://cloud.githubusercontent.com/assets/6137919/3635734/b39ff99a-0f87-11e4-93fd-7d9793e47c9b.gif)
This will cause `pytyle3` to crash if you haven't updated your configuration. I'd like to avoid that.

Could you change references like `config.gap_change` to `getattr(config, 'gap_change', 5)`? Similarly in other places.