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)
Writing to a config `gap` variable is strange. It seems like you're abusing it for global state, which means the changes will leak over into other active layouts.

Could you use `self.gap` instead of `config.gap`? This way, the gap is local to each instance of a layout.