merge tutorial into api doc

tutorial/html-custom-element.md

-# HTMLCustomElement
-
-This interface class extends the native `HTMLElement` interface and
-provides some additional, often used methods.
-
-## Methods
-
-### waitForElementById(id)
-
-equivalent of native getElementById, but works asynchroneous and
-returns a Promise which resolves when the requested element is
-aviable.
-
-**Parameters**
-- id {string}: id of the requested element
-
-**Return**
-- {Promise<Node>}: resolve contains reference to the requested element
-
-### addAttributeListener(node, attrName, fn, IMT)
-
-Like an event listener but for attributes. It observes an elements
-attribute for changes and invokes a pre defined callback function. For
-this js MutationObservers are used.
-
-**Parameters**
-- Node {Node}: reference to the element
-- attrName {string}: name of the attribute to observe
-- fn {function(node: Node, attrName: string)}: callback function
-  invoked at attribute change
-- IMT {bool}: if `true` fn is executet immediately after listener is
-  registered

tutorial/installation.md

-# Installation

tutorial/tutorial.json

-{
-    "installation": {
-        "title": "Installation"
-    }
-}