Refactor subprocess call to pandoc

Changed call to check_call and passed in the args as a list object
instead of a formatted string.  Should make passing in options from
argparse easier.
Refactor test_importer

All of the tests were coercing the wordpress fields to list so I just
do it in setUp. DRY, etc.
Add decode_wp_content to pelican_import.py

This function is a near-direct translation of the function used in WP
itself to decode post content.  This allows the importer to create a
faithful HTML representation of the post to pass to pandoc for
translation to the output format.
Fix case for embeds in decode_wp_content
Add test case to importer for <li><pre><code></code></pre></li>