Add LibEvent submodule

This submoule points to the personal fork by @bassosimone, adapted to work
properly with Android NDK.
Adapt Android.mk to Libight file system

Libight has a different file system than LibNeubot, so I had to change the path
of the source files.

Moreover, this Android.mk includes the Android.mk of LibEvent, in order to build
it as well, during the compilation of Libight.
Add Application.mk to build using Clang, libc++ and C++11

Since GCC does not implement non-static data member initializers,
we decided to use Clang.

Since Libight uses some features of C++11, we have to build using this standard.
Merge pull request #15 from alessandro40/add-libevent-20140711

Adapt Android.mk to Libight and add LibEvent submodule