Adapt Android.mk to Libight and add LibEvent submodule

Since the gcc bundled with Android NDK r9d was not able to compile the assignment-style class members initialization used in Libight, we decided to use Clang (it can be done using the proper flag in the Application.mk file).

I added the LibEvent submodule, pointing to the personal fork by @bassosimone. This fork is properly edited to work with Clang and the Android NDK.

While there, I adapted the LibNeubot Android.mk to work with Libight file system.

I successfully built this version of Libight using Android NDK r9d.