Make integrative test run as during phase verify.

Right now java-gitlab-api fails during Maven phase test when no
GITLAB instance is running on localhost.

Make this test run during `mvn verify` only and check beforehand
whether the GITLAB instance is reachable. Furthermore the
instance-under-test may be changed by adding two system properties
during invocation.

Running:
```
mvn clean verify -DTEST_TOKEN=ADMIN_TOKEN -DTEST_URL=https://GITLAB_URL
```
will pick up the values and use this instance/token.
`assumeThat` not working as expected, throw an AssertionError when
we do not get an `401 Unauthorized` during `setup`.
Merge pull request #29 from mfriedenhagen/integration-tests

Make integrative test run as during phase verify.