Fixed: output handling & coloring

Hi!

I have two bugs with mix process output handling:

1. A lot of terminal control characters in output
2. [This](https://github.com/tonini/elixir-mix.el/blob/master/elixir-mix.el#L193) code is running too early (because compilation is asynchronous)

![screen1](https://cloud.githubusercontent.com/assets/1789390/4098889/d095b16c-303d-11e4-8f27-7c131e6ea34e.png)

This commit fixes both issues:

![screen2](https://cloud.githubusercontent.com/assets/1789390/4098890/d2b5b820-303d-11e4-8ac7-118a7c4836ba.png)