This closes #232

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.

Users using CDH4 can now build Flink against Hadoop 2.0.0-alpha.

[Documentation] Move "How to contrib"; "coding guidelines" out of the code repo (part of FLINK-1012)
Improve build instructions with respect to Hadoop 1 and Hadoop 2 dependencies.
[ci skip]

This closes #62