Enhancement: add Fluent-API to relevant setters in the model classes

[mivola]

The setters in the model classes (i.e. BaseModel, Build, Job etc) should follow the Fluent-API pattern and return the object itself instead of void to allow the client calls like "artifact.setJenkinsServer(server).setClient(client);"

[khmarbaise]

Please squash your commits into a single commit and fix the commit message like this:

Fixed #360 
 - Optional explanation

That will help to close the issue automatically if it's merged to master.

[mivola]

@khmarbaise tbh I'm not sure which is the best way to do the squash - can you give me a short explanation?

[mivola]

@khmarbaise as I couldnt find a way to squash the commits in a remote branch, I created a new branch and a new PR: #380. As an alternative: couldnt you squash the commits of this PR when merging it into master?