Change default halo.png for better visibility.
Make config compatible with C++11
Add a .mailmap file

So I was checking out minetest and wondered how many developers there are
and how they development community is setup and such, so I decided to have
a look at the git shortlog. There were lot of double entries for some
persons, so it was harder to estimate the number of people involved.

By adding a .mailmap file, the overview of the authors is much improved
For details on the .mailmap file, see
https://www.kernel.org/pub/software/scm/git/docs/git-shortlog.html

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
Small changes to French translation (Petites modifications mineures)
Fixed french vocabulary/grammar errors

Notes (in french) :

72 msgstr "Ok" : OK est un acronyme, pas un nom, donc majuscules.
193 msgstr "Feuilles transparentes" : quel type de feuilles ? Trop vague.
213 msgstr "Filtrage anisotropique" : oxymore.
229 msgstr "Précharger les visuels d'objets" : doit être écourté (dépasse trop la bordure grisée)
Revert "Small changes to French translation (Petites modifications mineures)"

This reverts commit 4432e982616cd871cf1ae5c5706da09e98077433.

Exception: "Nom / Mot de Passe" --> "Nom / Mot de passe"
.travis.yml: fix possible issues

* use `pool.sks-keyservers.net` as keyserver as it's often more reliable
than `keyserver.ubuntu.com` as you can see with Google.
* put script to multiple lines
    * build out-of-source
    * use two CPUs
        * make seems to fail if $(nproc) is used unlike WeeChat. ZNC has
          same issue.
* mark build as failed if one job fails immediately without waiting for
all builds to finish.
    * this doesn't stop jobs that didn't finish before build was marked
    failed.
Add compression API
Make getters of the Settings class const

Also removed 2 unused functions.
Use const references for Settings methods

Also check for (this == &other) before locking mutexes.
Fix Settings locking
Split settings into seperate source and header files

This also cleans up settings a bit
Fix locking bugs and make inventory deserialization errors more specific