Upgrade App Engine SDK to 1.8.8

Upgrade the App Engine SDK to the latest version (1.8.8). This commit
basically just changes some JAR files in `war/WEB-INF/lib`.

Developers working on the application after this commit are expected
to have version 1.8.8 of the App Engine SDK installed, and have
Eclipse configured to use it.
Upgrade App Engine testing utilities to 1.8.8

Update the classpath to use the App Engine testing utilities for
1.8.8.

For more information on the App Engine Java testing utilities, see:
https://developers.google.com/appengine/docs/java/tools/localunittesting#Java_Introducing_the_Java_testing_utilities