Adds android example using jni.

examples/cross_calculator/android/readme.txt

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+In this directory you will find the Android platform cross-calculator sample.
+
+Due to the nature of Android being java and nimrod generating C code, the build
+process is slightly more complex because jni code has to be written to bridge
+both languages. In a distant future it may be possible for nimrod to generate
+the whole jni bridge, but for the moment this is manual work.
+
+For the jni bridge to work first the java code is compiled with the nimrod code
+just declared as a native method which will be resolved at runtime. The scripts
+nimbuild.sh and jnibuild.sh are in charge of building the nimrod code and
+generating the jni bridge from the java code respectively. Finally, the
+ndk-build command from the android ndk tools has to be run to build the binary
+libary which will be installed along the final apk.
+
+All these steps are wrapped in the ant build script through the customization
+of the -post-compile rule. If you have the android ndk tools installed and you
+modify scripts/nimbuild.sh to point to the directory where you have nimrod
+installed on your system, you can simply run "ant debug" to build everything.
+
+Once the apk is built you can install it on your device or emulator with the
+command "adb install bin/CrossCalculator-debug.apk".
+
+You can use this example as a starting point for your project or look at the
+history of the github project at https://github.com/gradha/nimrod-on-android.
+That repository documents the individual integration steps you would take for
+any Android project (note it uses Eclipse rather than ant to build and
+therefore the build process requires more manual fiddling).
+
+This example runs against the Android level 3 API, meaning devices from
+Android 1.5 and above should be able to run the generated binary.