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.

To build succesfully, I had to remove the file bufferevent_openssl.c from the Android makefile, since it did not find an OpenSSL header.

Moreover, I reverted the commit 408fa9c, because it is no more needed.

I succesfully built LibEvent using android-ndk-r9d ndk-build.