Add some files
Add some files.
Add blank README.md
Fix #18.
Comment out script line of .travis.yml
Fix .travis.yml
Modify get_descendants algorithm. .travis and some tests.
Add -v to .travis.yml before_script
Fixing
Add integration_test to make test
Fix Makefile
Fix make test failure.
Fix .travis.yml
Create tmp directory if it does not exist
Fix .travis.yml.
Fix error code of tree_db --help
Fix .travis.yml
Intentionally fail to TravisCI for .travis.yml test.
Revert "Intentionally fail to TravisCI for .travis.yml test."

This reverts commit 80f37d50ea16bef519299ba29c7a0349e30c6ec2.
Setup QuickCheck test.