Small code refactoring

The purpose of this refactoring is to make code more pretty and more readable.

The refactoring includes next changes:

* Replaces obsolete `fix_siteurl` with new `normalize_url`
* Add tests for utils module.
* Add tests for markdown converter.

Fixed #12
Merge pull request #14 from ikalnitsky/refactoring

Small code refactoring
Implementation of the blog generator

This commit adds implementation of the blog generator, which
incorporates feed, index page and tags pages generators.

Fixed #1