API: List issues for a particular milestone

# What does this MR do?
This MR adds a new API method for retrieving the issues associated with a particular milestone. The resource is described at ":id/milestones/:milestone_id/issues", returning a simple array of issues.

I've picked up gitlab-org/gitlab-ce!113  and fixed the test for it.
From the original MR:
This feature was motivated by an in-house project for reporting--for large projects, retrieving and filtering the entire issue list by milestone was becoming too slow, particularly when pagination required multiple requests. While this resource also uses pagination, it allows the consumer to limit reporting to the particular milestone they are interested in.

@dblessing Please take a look :)

See merge request !308