Test red green cycle

hey @pragdave

First, thanks for your great idea about the red/green automated test cycle. :+1: :yellow_heart: :green_heart: 

I tested the whole thing for a bit today and found a few things:

* I'm not sure about if the automated test run should be enable as default, but thats something we could think about it.
* I added the `after-save-hook` just for file which are in the `elixir-mode` major mode. (`*.exs, *.ex` files)
* I renamed the `alchemist-on-save.el` to `alchemist-hooks.el` maybe you have some more nice ideas about hooks in the near future. ;-)
* I thought it could be nice to have a nice information in the `mode-line` about the elixir test suite status, so I added two different faces for `success` and `failed`.
* The `mode-line` will be initialized when you use `(alchemist-mode 1)` and will be reset to the users default `mode-line` when `(alchemist-mode nil)` will be set.
* There is a `alchemist-mix-display-mix-buffer` function now, so ppl could always display the test buffer when they need some more specific informations about the test run.

The whole thing need some more documentations for the variables, functions etc. Also the README must be updated. But first I thought it would be great If you could try out the whole thing by yourself and maybe give some feedback.

Thanks a lot for your contributions.

Sam