Support chained fetching multiple URLs

@dpup,

I need to make falkor support fetching multiple URLs in order, in the meantime, I want each URL to have its own parameters and expectations.

This PR is a prototype and I will be working on test cases. As I'm new to javascript, I'm actually not 100% sure what I'm doing. Given that, I would rather submit a PR asap so you can take a look. Maybe you have better ideas.

Basically what I did is to generate one TestCase for each URL, so each of them can have separated parameters and expectations. Each TestCase instance has its own noteunit-like wrapper. A chained testcase will return the wrapper of the last fetch. When it executes, I recursively fetch URLs in their original order.