Mocking of JenkinsClientJobs, JenkinsClientBuilds, etc.

[IlyaAtGitHub]

IJenkinsClient interface has JenkinsClientJobs and other properties properties defines as classes and not as interfaces. It prevents from mocks creating in Unit Tests. I could work on PR for to add interfaces to JenkinsClientJobs, JenkinsClientBuilds, JenkinsClientQueue and JenkinsClientArtifacts if it's OK for you.

[myarichuk]

Probably related: #54