`NewFakeClient` should accept objects to be available in the hash tables

[ricardozanini]

Some times during tests we wish to have objects created in the in memory database before hand. A default Nexus server comes with some default repositories such as maven-central. The Fake client could also have these entries on creation or accept such objects in the constructor. Something like:

nexus.NewFakeClient().
    WithMavenGroups(...).
    WithMavenProxies(...).
    WithUsers(...)