Added REST endpoint for an overall summary of the Cluster.

This provides a REST endpoint for an overall cluster overview.  It will fuel the xhr interactions with the primary dashboard.

Example response:
HTTP 200 OK
Vary: Accept
Content-Type: text/html; charset=utf-8
Allow: OPTIONS, GET

{
    "status": {
        "osds": {
            "warn": 0, 
            "crit": 0, 
            "ok": 2
        }, 
        "mons": {
            "warn": 0, 
            "crit": 0, 
            "ok": 1
        }, 
        "pgs": {
            "warn": 192, 
            "crit": 0, 
            "ok": 0
        }
    }, 
    "activities": {}, 
    "osd_states": {
        "exists+up": 2
    }, 
    "health": {
        "clusterHealth": {
            "status": "HEALTH_WARN", 
            "statusDescription": "WARN"
        }
    }, 
    "usage": {
        "clusterDataTotal": "14.0", 
        "clusterBytesTotal": 15009275904, 
        "clusterBytesAvail": 14931873792, 
        "clusterDataUsed": 0.1, 
        "clusterDataScale": "GB", 
        "clusterDataAvail": 13.9, 
        "clusterBytesUsed": 77402112
    }, 
    "pg_states": {
        "active+degraded": 192
    }
}