[WIP] Use JAX-RS and resteasy for implementing the GitLab API

[coder-hugo]

This PR replaces the java-gitlab-api dependency with an interface annotated with JAX-RS annotations that only "implement" the required methods of the GitLab API. Based on this annotated interface resteasy can create a client as a Java Proxy for the interface.
Each method invocation of this interface will end in a single HTTP call (this is different in the java-gitlab-api) which makes it more easy to test if the plugin will call the GitLab API correctly.
Fixes #212

[jenkinsadmin]

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[omehegan]

@coder-hugo just wondering what the status of your work on the 1.2 release is.

[coder-hugo]

@omehegan I was very busy for the last 3 weeks so I hadn't any time to work on the plugin.
From my point of view the branch release-1.2 should by ready for a release. I just wanted to add some further integration tests. But they aren't necessary for a release. If I find some time this evening I'll either merge the release-1.2 branch to the current master or just make it the current master. If this is done we can do a new release.