* [WIP] Implement automatic spell-check for docs
fixes#663
The only thing unclear to me so far is how the output of `make spelling`
should be checked - is there a possibility to check for a file (i.e.
`_build/spelling/output.txt`) to be empty, and report a failed build
otherwise?
* fix typo in requirements.txt
* add enchant library
* travis should report errors, order spelling wordlist
* change travis.yml to easier troubleshoot build issues
* fixed more typos, added more words
* add more words, fix more typos
* added more words
* added more words
* revert changes to .travis.yml
* add basic instrumentation possibilities to pretix
* make tabs to spaces
* apply flake8
* implement upstreams suggestions, round 1
* adjust naming of redis-connection
* address noredis
* add view for metrics
* implement HTTP basic auth in front of metrics-endpoint
* rename labelset
* make flake8-clean
* implement upstreams suggestions, round 2
* correct minor slipups
* fix missing return
* let isort add an empty line
* implement test for counter
* implement upstream suggestions, round 3
* correct typo
* implement first test for view
* finish view-test
* fix deprecated keyword
* implement upstream-suggestions, round 4
* implement test for gauge
* test exceptions as well
* add db-decorator
