Implement c++11 functions that aren't supported on Android

Android NDK toolchain doesn't have some functions from C++11 (I fixed std::to_string and std::stoul). As far as I understand it is caused by using bionic instead of glibc.
I implemented such functions for Android by means of std::stringstream. This patch makes it possible to compile Botan using Android NDK. I tried to use Botan on Android ARM devices and found it working now.