Made a generic stub server and refactored DefaultCourseServiceTest to use it
Tests for DefaultExerciseService
Tests for DefaultInstanceService
Tests for DefaultSnapshotFileService
Merge branch 'master' of github.com:kesapojat/codebrowser-backend

Conflicts:
	src/test/java/fi/helsinki/cs/codebrowser/service/DefaultCourseServiceTest.java
Code style