Fix #73: Add default configuration for CircleCI using Gradle

[BenHenning]

This fixes #73 by setting up CircleCI using Gradle. This configuration will need to change or we'll need to adopt a different service once #59 is resolved.

This includes adding the Gradle wrapper to a special .circleci directory for the specific purposes of .circleci (we do not include the Gradle wrapper in the top-level directory as is customary for Gradle projects). gradlew was generated by Gradle, but partially modified to support not being in the project root directory.

[BenHenning]

See the CircleCI output--tests appear to be running correctly and reporting to GitHub. This won't be marked as a blocking check yet. We need to fix existing broken tests before we can do that. #295 is tracking this.

[BenHenning]

Also, note that I'm not sure if Espresso is actually running here. I expect the top-level gradle test to run the app tests as Espresso by default, but I'm not actually starting up an emulator (maybe Gradle does automatically?) This may require additional research as we work to fix the current broken tests.

[seanlip]

This looks fine to me, thanks @BenHenning! Feel free to merge.

[BenHenning]

Thanks @seanlip.

FYI @oppia/oppia-android that this is going in. It's not blocking, but you'll see it on your PRs. I'll make a broader announcement once this check is enforced.

[rt4914]

Thanks @seanlip.

FYI @oppia/oppia-android that this is going in. It's not blocking, but you'll see it on your PRs. I'll make a broader announcement once this check is enforced.

@BenHenning Thanks for the heads up.