Added a Allocation#for_job API

[tbartelmess]

Added a simple API to Allocation to fetch all allocs for a given job, wrapping the /v1/job/<ID>/allocations API endpoint.

An API like

Nomad.job('my_job').allocations

would probably be nicer, however a Response doesn't have access to the Client at this point, enabling an ABI like this would require a few more changes.

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