LibCore: Add a basic Core::DateTime class

This is just to have a pleasant way to print the current time for now:

    dbg() << Core::DateTime::now();

Or if you want it as a string:

    Core::DateTime::now().to_string();

Libraries/LibCore/Makefile

@@ -1,5 +1,6 @@
 OBJS = \
     ArgsParser.o \
+    DateTime.o \
     IODevice.o \
     File.o \
     Socket.o \