added extractor interfaces

This will be the first of several commits trying to create a more
high-level and abstract API. Some hefty refactoring is about to follow.
starting to move html package into encoding package
copied Article and Chunk to enc package
removed init function
removed html package
renamed package model to ml
added implementation of the Article struct
porting the ml extractors to the ext interfaces
added Link implementation
added Robots.txt parser
added RSS, Sitemap and Atom parser
added tests
added small networking layer on top of http.Client
laxer charset handling
ignore comments
ingore directives as well
added decompress function
fixed typos and simplified test code
added special link type
use integer codes to compare tags