Issue18 - Reftest docs

(1) "reftest-main-tutorial.md" is the entry point, which is based on Dzenana Trenutak's work. 2 templates are also referenced here: "template-reftest-001.html" and "template-reftest-002.html".

(2) This main tutorial also links to a more detailed documentation "reftest-doc.md", which is based on CSS Test Wiki's documentation @http://wiki.csswg.org/test/reftest. More examples (.html and .xht) and selftest.md are also referenced there.