Added method to get all commits for a project

Make integrative test run as during phase verify.

Added deleteGroup, deleteProject

Added GitlabProject support for last_activity_at and archived properties

we do not get an `401 Unauthorized` during `setup`.

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.

added addGroupMemeber, deleteGroupMember, addProjectMember, deleteProjec...

prep support for https://gitlab.com/gitlab-org/gitlab-ce/issues/483

Diffs, Project Slugs and a couple other things

Added gradle wrapper

support for user CRUD

readding the json properties

fixing up code style to match master

trying to fix up whitespace issues to match original changes

white space fixes for gitlab branch and project hook

clean up gitlab api file

fixing compile issues with gitlab api

fixing compile issues with gitlab api

added getUsers() and getUser( userId )

Added gradle wrapper so no local gradle installation is required and the used
gradle version can be set by the build file.

added gradle build file