Include caffe2 as submodule

[andyli]

It removes the use of pre-built binaries, such that the whole application will be built from source together with the same configuration (e.g. ANDROID_STL, ABI etc).

It also allows building with newer Android SDK (fix #55).

Notice that I added a testing keystore file to be used for building the app in release mode. That's because when building for debug, the native lib will also be build in debug mode, which is much much slower than a release build.