add octopress-escape-code gem

Add the octopress-escape-code gem to remove the need to surround code
blocks with {% raw %}/{% endraw %} tags when they contain liquid code
examples
modify backtick blocks to hide linenumbers

In vanilla Octopress, backtick fences and `{% codeblock %}` are pretty much the
same, and at least with Pygments neither supports suppressing line
numbers.

This modifies backtick fences to suppress line numbers, leaving `{%
codeblock %}` with their default behavior.
changes to related posts section in post layout

Updates the header for related posts from "Read More", which is
potentially misleading (it can signify pagination), to "Related Posts".
Also increases the number of related posts from 3 to 5.