feat(suite): bind this to the current test

Bind this to the current test at execution.
Semantic: "this is the current test".

No need to use a function argument for accessing DSL.

Example Code:
```js
module.exports = {
    // Checks if the <title> of ´github.com´ has the expected value
    'Page title is correct': function () {
        'use strict';
        this.expect(1);

        this
            .open('http://github.com')
            .assert.title('GitHub')
            .done();
    }
};

```


Also set test via the arguments array to backward compatibility.


I've also tried to add a test for this, but there is not so much base-tests to extend ...
So I'm not so deep enough into dalek to create the initial test-suite.