Remove example _posts directory

This is a documentation site so we're not using posts at this stage.

This also fixes the following warning message:

Build Warning: Layout 'post' requested in
_posts/2014-04-14-welcome-to-jekyll.markdown does not exist.
Rename pygments option to highlighter in config

This fixes a jekyll warning message.
Link Get started button to documentation

And remove the Documentation link on the homepage.

Fixes #29