Implement automatic spell-check for docs (#688)

* [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

.travis.yml

@@ -36,5 +36,10 @@ matrix:
       env: JOB=tests PRETIX_CONFIG_FILE=tests/travis_postgres.cfg
     - python: 3.6
       env: JOB=plugins
+    - python: 3.6
+      env: JOB=spelling
 addons:
   postgresql: "9.4"
+  apt:
+      packages:
+          - enchant