Better comments in Quantity class.
Added `has_url_any_extension()` method to utils/url.
Added Link class.
Added `unique_list()` method to utils/python.
Added unittests for `flatten()` method.
Moved test_selector into test_parser dir.
Added `safe_urlparse()` method to utils/url.
Use `request.parsed_url` in Policy class.
Moved http tests to dir test_http/.
Optimized `replace()` method in TextResponse.
Added unittests for TextResponse.
Added `BaseLinkExtractor` class.
Added `LxmlLinkExtractor` class.