Load setup files instead of sourcing them

- needed for same setup files to work on Windows
  (where PowerShell is used instead of bash)
- removes ugly bashism to allow overriding from .travis.yml
- adds a proper (yet small) syntax
Also adds tests for the new syntax and updates the README.

test03.set

@@ -0,0 +1,4 @@
+# Check for avoiding multiple includions
+
+include test01
+include test01