Enabling Markdown and other JSDoc improvements

This change enables GitHub flavored markdown for APIdoc
comments. The code example in map.js shows how to use markdown
for code snippets. doc/index.md is now included again as start
page for the docs. Everything in the doc/tutorials directory
will also be added to the docs as tutorial. As an example, I
moved the ol3.md file with the architecture to the tutorials
directory.

Currently properties and methods annotated with @inheritDoc
or @override won't be documented at all. This is a known issue,
so I added a custom JSDoc plugin with a hack to avoid this.

doc/plugins/inheritdoc.js

@@ -0,0 +1,16 @@
+/*
+ * This is a hack to prevent inheritDoc and override tags from entirely removing
+ * documentation of the method that inherits the documentation.
+ *
+ * TODO: Remove this hack when https://github.com/jsdoc3/jsdoc/issues/53
+ * is addressed.
+ */
+exports.handlers = {
+
+  beforeParse: function(e) {
+    e.source = e.source.replace(
+        /\/\*\*\r?\n?\s*\* @(inheritDoc|override)\r?\n?\s*\*\/\r?\n?/g,
+        "/***\n *\n */\n");
+  }
+
+};