Added: test coverage with undercover.el

Hi!

I have released a new Emacs library [undercover.el](https://github.com/sviridov/undercover.el) that provides a test coverage check for Emacs packages.

This commit adds [undercover.el](https://github.com/sviridov/undercover.el) to emacs-elixir. [Check](https://coveralls.io/r/sviridov/emacs-elixir) my fork coverage results.

Somebody should enable elixir-lang/emacs-elixir for [Coveralls](https://coveralls.io/).

Screenshots:

![1](https://cloud.githubusercontent.com/assets/1789390/4490597/14a2dc0e-4a2e-11e4-9816-5ca256dac7a4.png)
![2](https://cloud.githubusercontent.com/assets/1789390/4490598/14a34176-4a2e-11e4-97c4-e030db708c98.png)