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.sh

@@ -25,6 +25,14 @@ if [ "$1" == "doctests" ]; then
 	cd doc
 	make doctest
 fi
+if [ "$1" == "spelling" ]; then
+	XDG_CACHE_HOME=/cache pip3 install -Ur doc/requirements.txt
+	cd doc
+	make spelling
+	if [ -s _build/spelling/output.txt ]; then
+		exit 1
+	fi
+fi
 if [ "$1" == "tests" ]; then
 	pip3 install -r src/requirements.txt -Ur src/requirements/dev.txt -r src/requirements/py34.txt
 	cd src