Add support for namespace in get_job

Similiary to https://github.com/jrxFive/python-nomad/pull/115, I'd like support for passing the namespace to the get_job function. The API already supports it: https://www.nomadproject.io/api-docs/jobs#read-job

This way I can connect to the nomad-API with a specific namespace but can also get jobs from other namespaces.