add notes how to deploy to heroku
adds dummy extractor

The extractor simply generates a message saying that sitebag was
unable to extract text from the given document. This is useful if the
original document should nevertheless be stored in sitebag, even
without content extraction. As more extractors may come in over time,
extraction could be retried later on this document.
make proxy config visible in sitebag.conf
fixes json in "untag all" request
Merge branch 'master' into heroku