Move of documentation to libraries - first round

Hi Evan.

Here is the first round of moving docs from elm-lang.org to the library files. I have left out the areas you mentioned here https://groups.google.com/d/msg/elm-discuss/xHVCb7Rq8pM/9nIPmj9VHZoJ

For descriptions that I didn't find any definition for in the new libs I have added a comment section for you to decide what to do with them. Just grep for TODO. In the more trivial cases I have just added missing definitions (if I found the definition was still in the Native lib's).

I have done some updates in the Json.elm lib descriptions in an effort to match the new naming / structure - please review (also marked TODO).

I have moved "intro" descriptions to the head of the modules - "attaching" them to the module definition line. That way you can modify the elm-docs to pick them up as module doc strings. Likewise I have moved docs for type/data definitions where they existed, again you could add the extraction of these to elm-docs if you think it makes sense.

Mads