Update to use latest Gradle and SDK versions

This pull request is essentially a maintenance update. Updates Gradle version to 12.2 (currently can't compile in the latest version of Android Studio without it) and SDK target/compile versions to 20.

A couple of other little things were updating the plugin declaration to the newer `com.android.application/library` format and suppress a `serializable` warning that would fail tests.

Speaking of tests, [this](https://gist.github.com/pandanomic/f15debefc2a0d5538aa1) is the current output of running `build_and_test.sh`. 7 unit tests at the end fail, but I don't know if that was the case before since it wouldn't get that far before I suppressed the serializable issue in 7635194.

I filled out the CLA, let me know if there's anything else I need to do!