travis: build debian packages and documentation

The resulting packages are pushed to Debian repositories hosted on
bintray.com.

This is the first step to move away from our custom buildbot setup (see
https://i3wm.org/docs/buildbot.html for details on that) towards
infrastructure which is more standard (travis) and in the open.

travis/check-formatting.sh

@@ -1,2 +1,6 @@
 #!/bin/sh
+
+set -e
+set -x
+
 clang-format-3.5 -i $(find . -name "*.[ch]" | tr '\n' ' ') && git diff --exit-code || (echo 'Code was not formatted using clang-format!'; false)