As a sysadmin, I want to have a healthcheck resource to Crowd

This change adds a health check resource at:

http://localhost:8095/crowd/rest/healthcheck/1.0/check

that will return a successful 200 (OK) or 204 (No content) to indicate that Crowd is running.

Any registered internal healthchecks may cause this to fail, and details of the problems will be included in the response.