Merge remote-tracking branch 'origin/79-rename-amies-into-directmairie-in-documentation' into 79-rename-amies-into-directmairie-in-documentation

It's still very ugly, but not as much as the previous one...

The property is now used in the mailing mustache templates, and is also used by default in the from email addresses.

Note:
 - a typo has also been fixed in one of the templates
 - missing validation has been added for the root directory property (so the error will be clearer if it's forgotten)

fix #63

fix #60

also fix a bug in the secret key generation class

also fix an exception-related bug noticed by documenting the code