update media/file columns to match drupal migrate 2.5
quick changes, need rework

support files from different domain that host, eg mysite.example.com can
use files cdn.example.com--needs to be reworked tho

URI.escape is deprecated, use WEBrick::HTTPUtils and do a quick sanity
check to avoid most double-encoding issues
add note about nokogiri libxml2.9 issues and minor copy changes