added checkpoint info to serverStatus

#1076 

Example:
```
	"checkpoint" : {
		"count" : 11,
		"time" : 4,
		"last" : {
			"begin" : ISODate("2014-04-21T23:23:18Z")
		},
		"lastComplete" : {
			"begin" : ISODate("2014-04-21T23:23:18Z"),
			"end" : ISODate("2014-04-21T23:23:19Z")
		},
		"begin" : {
			"time" : 782
		},
		"write" : {
			"nonleaf" : {
				"count" : 17,
				"time" : 0.0000617393103448276,
				"bytes" : {
					"uncompressed" : 12697,
					"compressed" : 15360
				}
			},
			"leaf" : {
				"count" : 89,
				"time" : 0.008541220689655172,
				"bytes" : {
					"uncompressed" : 245506872,
					"compressed" : 31551488
				}
			}
		}
	},
```