FAQ: error message when RabbitMQ is not running

Fixes #46.
doc: xtas.celery renamed xtas.core

Spotted by @mariahendrike.
tutorial: format renamed "output"
improve clustering task docs
much more detailed installation instructions

Thank you, hackathon folk!
credit @dodijk, @nusselder, Tom
Add ILK Frog task (called via socket)
move Frog docs so they show up in the manual
introduce xtas.worker command instead of celery -A xtas.tasks

Allows us to do custom stuff at startup, like log where we found the
configuration and what it contains.
improved logging/report settings at startup

Fixes #42, #47.
xtas.worker --version
Added dbpedia_spotlight() single task
credit @aolieman
clarify Stanford NER download
run chardet from fetch function and do proper typecheck
pep8 DBpedia Spotlight test
fix UnicodeEncodeError in stanford_ner

When data in elasticsearch contained a unicode character, a UnicodeEncodeError
was thrown. nltk tokenised text is now explicitly encoded as utf-8 before
sending the text to stanford ner.

Fixes #51.
increase test verbosity on Travis