Initial commit for Ads Service. (#21)

* Initial commit for Ads Service.

* update comments for AdsService and AdsServiceClient

* Refactor Ads to Ad
Move building AdService to Docker
Use default setting for Stackdriver Exporter.
Add license text.

* Revert the projectId

- also remove commented code from frontend/rpc.go

* Add adservie to skaffold.yaml

* Remove skaffold-adservice.yaml

* Replace personal projectId with demo projectId.

* Fix the crash in adservice when ran in locally.

* Ignore .skaffold*yaml file and .kubernetes-manifests-*/ dir for easy ProjectID switch.

* Fixed review comments.
1. Changed Ad redirect urls to products.
2. Removed leftovers from Dockerfile/kub*manifests*yaml
3. Added retry for StackDriver.
4. Added log for Ad request.
5. Added comment for gradle caching.
6. Added README.md to src/adservice.

* Added GRPC Health service to Ad Service
Also added
1. timeout to getAd RPC call in frontend.
2. Async thread for stackdriver init.

pb/demo.proto

@@ -218,18 +218,18 @@ message PlaceOrderResponse {
     OrderResult order = 1;
 }
 
-// ------------Ads service------------------
+// ------------Ad service------------------
 
-service AdsService {
-    rpc GetAds(AdsRequest) returns (AdsResponse) {}
+service AdService {
+    rpc GetAds(AdRequest) returns (AdResponse) {}
 }
 
-message AdsRequest {
+message AdRequest {
     // List of important key words from the current page describing the context.
     repeated string context_keys = 1;
 }
 
-message AdsResponse {
+message AdResponse {
     repeated Ad ads = 1;
 }