Add Gradle command-line build system and ProGuard optimizations.

I got fed up with Eclipse, so I investigate command-line build systems for Java and Android. Gradle is Android's new official build system. People can still use Eclipse if they like.

http://www.gradleware.com/resources/tech/android

http://tools.android.com/tech-docs/new-build-system/user-guide

I also enabled ProGuard optimizations for release builds. ProGuard shrinks our MozStumbler.apk from 258 KB to 70 KB. I didn't enable ProGuard obfuscation because we don't care about hiding our open source code and obfuscation makes exception stack traces more difficult to debug.