Add Datastore indexes samples. (#214)
Add Datastore indexes samples. (#214)

Samples are moved from:
https://cloud.google.com/appengine/docs/java/datastore/indexes

Note: I add a new script `test-devserver.sh` to the testing config. This
script runs the `mvn appengine:devserver` plugin, waits for it to start,
then verifies that it gets a non-error response from the `/` path. I use
this to verify that the `datastore-indexes.xml` files are correct (by
disabling autoGenerate, the local devserver throws an error when a query
is used without the correct index defined, just as production does).

We should probably add any "hello world" or other projects to this
check, as well. Anywhere we say to run `mvn appengine:devserver`, it
would be good to test that it is correct.
GoogleCloudPlatform/getting-started-java,GoogleCloudPlatform/appengine-try-java,tswast/java-docs-samples,xavierlepretre/googleCloudHackathon,jetty-project/java-docs-samples,xavierlepretre/googleCloudHackathon,ludoch/getting-started-java,GoogleCloudPlatform/java-docs-samples,skjhavit/GCPJavaProjectTrial1,GoogleCloudPlatform/appengine-try-java,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-repo-tools,Stevers146/appengine-springboot-gradle,skjhavit/GCPJavaProjectTrial1,tswast/java-docs-samples,GoogleCloudPlatform/getting-started-java,briandealwis/getting-started-java,GoogleCloudPlatform/java-docs-samples,Stevers146/appengine-springboot-gradle,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/getting-started-java,ludoch/getting-started-java,GoogleCloudPlatform/java-docs-samples,skjhavit/GCPJavaProjectTrial1,jetty-project/java-docs-samples,GoogleCloudPlatform/appengine-try-java,jetty-project/java-docs-samples,xavierlepretre/googleCloudHackathon,briandealwis/getting-started-java,ludoch/getting-started-java,tswast/java-docs-samples,jetty-project/java-docs-samples,xavierlepretre/googleCloudHackathon,tswast/java-docs-samples,GoogleCloudPlatform/java-repo-tools,GoogleCloudPlatform/java-docs-samples,jetty-project/java-docs-samples