Put everything in namespace 'flx'.

Everything now resides in the namespace 'flx'. Also, header files now
sit in the folder "include/Flexium/", so you have to include, for
example <Flexium/World.hpp> instead of just <World.hpp>.
Add minimalistic version of Console.hpp

Flexium/ConsoleMinimal.hpp provides a simple way to use to console
(flx::Console::Log), without the added weight of extra features that are
often not required.