tests: Implement Travis-based Multi-OS CI service

This commit implements (after the example of taskwarrior) a Travis-based
multi-OS continuous integration service, that tests on a variety of
Linux-based hosts and Mac OS X.

test/scripts/test_osx.sh

@@ -0,0 +1,18 @@
+set -x
+
+brew install gnutls
+brew install cmake
+brew install libfaketime
+git clean -dfx
+git submodule init
+git submodule update
+cmake -DCMAKE_BUILD_TYPE=debug .
+make -j2
+make install
+task --version
+
+pushd test
+make
+./run_all -v
+cat all.log | grep 'not ok'
+./problems