Adapt LibEvent to work with Android NDK

I forked the official repository of LibEvent. Then, I merged its `patches-2.0` branch with the same branch from the @ventureresearch repository, in order to apply their patches for building LibEvent on Android.

Since OpenSSL is not available, on Android, unless we compile it, I removed the file bufferevent_openssl.c from the Android makefile.

Moreover, I reverted the commit 408fa9c, because it is no more needed.

I succesfully built LibEvent using android-ndk-r9d ndk-build.