compiler requirements when Scipy/scikit-learn not installed

XXX we could make these optional, but then Gensim needs SciPy as well...
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
Fixed dbpedia_spotlight() output; added a few inline comments
Merge branch 'master' of https://github.com/NLeSC/xtas

Conflicts:
	requirements.txt
	xtas/tasks/single.py