Issue 19 - Documentation for testharness.js

testharness-documentation.md - extracted from the src
testharness-tutorial.md - Robin's doc. Note that I removed 2 paragraphs at introduction, and the Result section at the end. One paragraph about how this documentation is generated, which is not applicable any more. Another one about runnable code and test Result at the end after loading this doc, which is also not applicable any more.