[Cs3] recoded Command, updated forward declaration slides

[Mikhail Nesterenko]

CS3 students,

I recoded command example (that we studied in class) to remove
indexing operator overriding and to use std::stack to hold the command
history.  

I also updated Observer slides that introduced forward class
declaration to describe the kind of class declaration used in Memento
pattern example. It is called Elaborated Type specifier.

Please take a look,
-- 
Mikhail