Ability to call a function in the pre-require block

Adds the ability to pass a function in the task options that is called
in the pre-require block of the mocha runner.

I find that this can be useful to set up the mocha environment. For example, if I want to include Chai in all of my tests:

``` javascript
mochaWebdriver: {
  test: {
    options: {
      usePhantom: true,
      prerequire: function (context) {
        context.expect = Chai.expect;
        context.assert = Chai.assert;
        Chai.should();
      }
    },
    src: ['test/**/*.js']
  }
}
```

You could also just pass a module:
``` javascript
prerequire: require('test/setup.js')
```