Add the ability to skip tests - fix #403

Add test.skip:

```js
    casper.then(function () {
        test.skip(1, "Skip me.");
    });
```

And also:

```js
casper.thenSkip(1, "Skip me.");
// ....
casper.thenSkipIf(true, 1, "Skip me.");
// ....
casper.thenSkipIf(function () {
    return true; // check configuration or anything
}, 1, "Skip me.");
// ....
casper.thenSkipUnless(true, 1, "Skip me.");
```

I found `thenSkip*` quite useful to condition tests quickly.

Skipped tests are reported to test results.