Nokogiri XML Namespaces and gzip decoding

This is needed for fixing a few issues that DocumentCloud has encountered while using calais for our entity extraction.  

The first is that Calias sometimes returns gzipped content.  When that occurs an exception is thrown since the content can't be decoded (of course).   This may have been an intermittent issue with the api, but our thoughts were that it can't hurt to attempt to handle it.  A further enhancement would be to request gzip encoding on the request so it would be more efficient.

The second is more pressing.  It has to do with newer nokogiri differing on how it handles namespace prefixes.  I believe issues #10 and #11 are attempting to fix the same bug.  #11 indicates that the bug started with Nokogiri 1.5.6, but I haven't tracked down when the change occurred.

DocumentCloud has been running with this branch in production for several months now without issue (https://github.com/documentcloud/documentcloud/blob/master/Gemfile#L5).  We'd really like to get it merged and a new gem cut so we can remove the "git" references out of our Gemfile.

Thanks for the excellent job you've done with the gem thus far.   If I can help with any further testing or merging, please let me know.