Switching from Ack to Ag

I don't see why keep Ack next to Ag, so I made the full switch and deleted the old references to Ack (and included the Ag vim plugin in the README as well).

As side note: why does the zsh-completions project has the _ag completion file but I don't have it on my .yadr folder? The file is on their master for two months now and I installed yadr from scracth a few weeks ago, so why didn't the submodule get this file? Are we fetching from a different branch or something? https://github.com/zsh-users/zsh-completions/tree/master/src