Search for user specific config path and merge with the default config

Better guard against null values in user config
If the user set background cannot be loaded fall back to the default one
Fix build deps
fix crash on double pam handle free

fixes sddm/sddm#145
Merge pull request #146 from MartinBriza/pamcrash

fix crash on double pam handle free
Merge pull request #144 from davidedmundson/debian_build_deps

Fix build deps
Merge pull request #143 from davidedmundson/custom_background

Restore custom background support from sddm-kcm
Fix xauth annoying error
Merge pull request #147 from Teknocrat/xauth-fix

Fixes xauth warning about authority file not existing.
Add missing semicolon.
Don't set GDMSESSION as it is not used anymore.

Fixes #138.
improve .travis.yml

it's little bit messy, but it works
infrastructure is ready for clang, but for now build always fail
Merge pull request #149 from iXit/master

Fixes autobuilds using travis.
Added travis build status.