Adding new tests using mozhttpd server

Hi folks,

This PR is for discussion only. It is just the easiest way of sharing ideas.

I set up two very basic tests for the DirectoryParser and for the ReleaseScraper (-p win32 -v latest).

Note that there is a basic unittest class setting up mozhttpd and terminating it, so we just need to inherit from it.

`downloadable_tests` is the folder that mimicks the Mozilla Server, so we can just add downloadable content into it.

The things I would like to have your input in is:

- Current binary is real and too big. What are alternatives to this?
- Is the general set up ok like this or do you see obvious disadvantages?
- What are meaningful tests?
- Comments on code is always welcome.

Thanks. I'm looking forward to your input :)