Simpler inclusion of native components & support for x86

As far as I know, there is no need to package a native library in a .jar file; you can directly include the .so library files in the APK under `lib` by specifying `jniLibs.srcDirs = ['libs']`.

While I was at it, I added support for x86 processors; I believe there are at least some x86 Android smartphones out there (+ for development, there is an x86 Android emulator, which is faster to run on Intel PCs).

This will generate one APK with all native libraries inside. There is also the possibility to use Gradle flavors to create one distinct APK for each architecture; I can change the code to do that instead, if you want.

I also fixed some typos which were causing build warnings and updated to the lastest build tools.

*Note*: I thought you wouldn't like me checking in the binaries, so if you want people to be able to build without having the NDK installed (as it was the case previously), it is still necessary to build and check in the native components in `library/libs`. I added steps for that into `BUILDING.md`.