Android NDK toolchain support

[lasote]

• Add a toolchain for android
• android:settings command to help configure paths to android tools
• ...

[drodri]

Just realized that CMake > 3.1 has new support for Android: http://www.cmake.org/cmake/help/v3.1/prop_tgt/ANDROID_GUI.html
Maybe it is worth having a look. I am considering we should upgrade, as CMake 3.1 also has great support for compiler features & standards (C++11, etc)