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

src/pretix/base/models/orders.py

@@ -188,7 +188,7 @@ class Order(LoggedModel):
     def full_code(self):
         """
         An order code which is unique among all events of a single organizer,
-        built by contatenating the event slug and the order code.
+        built by concatenating the event slug and the order code.
         """
         return '{event}-{code}'.format(event=self.event.slug.upper(), code=self.code)
 
@@ -519,7 +519,7 @@ class AbstractPosition(models.Model):
     :type variation: ItemVariation
     :param datetime: The datetime this item was put into the cart
     :type datetime: datetime
-    :param expires: The date until this item is guarenteed to be reserved
+    :param expires: The date until this item is guaranteed to be reserved
     :type expires: datetime
     :param price: The price of this item
     :type price: decimal.Decimal