Multiple native architecture support 

This fixes the support for using multiple architectures where there is dependencies (shared/static) between native projects.

It uses the approach (currently) of two separate ndk-build invocations and gathering the artifacts from those instead of letting ndk-build do its thing entirely, as discussed/mentioned on the mailing list.

I deprecated the use of ndkArchitecture and added ndkArchitectures (plural) to signify the list of architectures to build for.

An error will be issued if the Application.mk defines more than one architecture via the APP_ABI variable.

Also introduces a mapping between architecture and toolchain to be used by the build:

 <ndkArchitectureToolchainMappings>
    <x86>x86-4.7</x86>
    <armeabi>arm-linux-androideabi-4.7</armeabi>
</ndkArchitectureToolchainMappings>

This can be used to specify the particular toolchain to be used for the individual architectures.

Thanks,

Johan