976089: Expose new api to get entitlements for a pool

This new API is at /pools/{pool_id}/entitlements. This API should be
more performant then iterating over all the entitlements.
Made changes to 976089 based on zeus' feedback
Merge pull request #306 from candlepin/bkearney/976089

976089: Expose new api to get entitlements for a pool