Alter logic for parsing solrconfig.xml.

Not sure quite what it was, but it was failing to properly determine
if dismax was available..
Be safer when trying to load XML.

... The given file might not have loaded successfully.
Fix the logic in checking the HTTP code, and fix a strict warning.
Avoid assignments in conditions.
Merge pull request #123 from adam-vessey/7.x-solrconfig-loading

Load and parse the solrconfig.xml, leveraging XPath to do most of the lifting...
Allow drush to work when in the islandora_solr module directory.

When looking back up the filesystem, Drupal bootstrapping hits a false
positive (http://drupal.org/node/1481270#comment-6585758 gave me the
clue)... So yeah, don't create an "includes/common.inc".
Merge pull request #125 from adam-vessey/7.x-fix-drush

Allow drush to work when in the islandora_solr module directory.