Add Cask and ert-runner. Tests refactoring.

Hi!

This commit adds support for [Cask](https://github.com/cask/cask) and changes some test related stuff.

## Cask and ert-runner

[Cask](https://github.com/cask/cask) is a project management tool for Emacs Lisp to automate the package development cycle.

### Installation

```bash
$ curl -fsSL https://raw.githubusercontent.com/cask/cask/master/go | python
$ cat >> ~/.bashrc
export PATH=~/.cask/bin/:$PATH
```

### Examples of usage

```bash
$ cd path/to/emacs-elixir
$ cask install # install ert-runner
$ EMACS=`evm bin emacs-24.3-bin` cask install # install ert-runner for Emacs 24.3
$ cask exec ert-runner # run tests
$ cask exec ert-runner -t 24-3 # run tests with 24-3 tag
$ EMACS=`evm bin emacs-24.3-bin` cask exec ert-runner -t 24-3 # run tests with 24-3 tag for Emacs 24.3
$ EMACS=`evm bin emacs-24.3-bin` cask exec ert-runner -t 24-3 --verbose # run it with elixir-smie-verbose-p=t
$ cask exec ert-runner --win # run tests with GUI window
```

## Some other changes

- `run_tests` script was removed.
- All the tests have been redefined with `elixir-deftest`.
- `elixir-mode-run-tests` was removed.