Enhancement: re-work crumbFlag handling

Currently the way to handle the crumbFlag is not very intuitive and even not complete. Some methods offer a parameter "crumbFlag" but other dont (see #350).

IMHO it would be better to handle this internally in the library or at least in one central place. So my suggestion is to have a new constructor:
`new JenkinsServer(..., boolean crumbFlag)`

Furthermore, if no crumbFlag is specified (by using the existing constructors) the library itself determines if the crumbFlag should be set or not by calling
```
String json = client.get("api/json");
boolean crumbFlag = new JsonObject(json).get("useCrumbs").getAsBoolean();
```

This would allow to cleanup the API from all overloaded methods that allow passing a "crumbFlag" parameter.

Start at the JenkinsServer constructors and the methods that currently expose a crumbFlag parameter. Trace the client.get("api/json") entry point and the existing overloads; done means crumb handling is centralized, an optional boolean constructor is available, and redundant crumbFlag parameters are removed.

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