[doc] Switch parser to kramdown, normalize Headings

The switch to kramdown is necessary because I want to add tabs in the
documentation for code examples and Redcarpet does not allow markup
inside divs.

Before, some doc pages had "#" headings as toplevel headings while
others had "##" (which is the same as --- underlined headings). Now we
user level 2 everywhere. The page title is still a h1 heading.
Add ExecutionEnvironment to ScalaAPICompletessTest
Rewrite Java API Guide as Unified Programming Guide

This now covers both Java and Scala.
[doc] Unify Examples Page

Now contains Java and Scala examples with tabs to switch between the
two.
Bump doc version to 0.7-incubating
Turn Documentation into standalone website, add Overview Page

This can now be built standalone and then copied into the correct docs
folder of the website SVN.

The index page now has a short overview and a table of contents.

                                                       tem
[scala] Fix Scalastyle violation in ExecutionEnvironment