Add index commit timestamp to the results

Now clients can display the index freshness when they render query results.

Sample:
```json
{
	"totalHits": 18220,
	"timeTakenInMs": 220,
	"indexTimestamp": "4/8/2014 11:54:21 PM",
	"data": [
		{
			"Key": 266057,
			"PackageRegistrationKey": 12653,
			"PackageRegistration": {
				"Key": 12653,
				"Id": "EntityFramework",
				"DownloadCount": 3420684,
				"Owners": [
					"microsoft",
					"EntityFramework"
				]
			},
			"Version": "6.0.2",
			"NormalizedVersion": "6.0.2",
			"Title": "EntityFramework",
			"Description": "Entity Framework is Microsoft's recommended data access technology for new applications.",
			"Summary": "Entity Framework is Microsoft's recommended data access technology for new applications.",
			"IconUrl": "http://go.microsoft.com/fwlink/?LinkID=386613",
			"IsLatest": false,
			"IsLatestStable": true,
			"Listed": true,
			"DownloadCount": 238374,
			"Created": "2013-12-13T18:56:02.267",
			"Published": "2013-12-13T18:56:02.343",
			"LastUpdated": "2014-03-12T00:11:24.213",
			"LastEdited": "2013-12-18T21:45:27.813",
			"Authors": "Microsoft",
			"Copyright": null,
			"Language": "en-US",
			"Tags": "Microsoft EF Database Data O/RM ADO.NET",
			"ReleaseNotes": null,
			"ProjectUrl": "http://go.microsoft.com/fwlink/?LinkID=320540",
			"FlattenedDependencies": "",
			"Dependencies": [],
			"SupportedFrameworks": [
				"net40",
				"net45"
			],
			"MinClientVersion": null,
			"Hash": "XcD+2UT/iEoJXCP1QB4yNJJPrPaIbBG0w0p2FVvGCs5GrPm3Bc2HKIsZFYj/gHoUayczyGE/lECd7vmp5QdaOg==",
			"HashAlgorithm": "SHA512",
			"PackageFileSize": 5035059,
			"LicenseUrl": "http://go.microsoft.com/fwlink/?LinkID=320539",
			"RequiresLicenseAcceptance": true,
			"LicenseNames": null,
			"LicenseReportUrl": null,
			"HideLicenseReport": false
		},
...
```