Added second task
Sandrikova Maria, 296, Calculator

Initial commit
Added Shell back on.
Added shell-specific exception class.
add ShellMain.java
error correction 1
not working cp; mv; package mode
working version 1
Added synchronizing with double checker for ShellReceiver singleton.
[Shell] Modified IO restrictions.
[Shell] Replaced Vector usages with ArrayList.
[Shell] Refactored 'cd' command implementation.
shell error correction 2
[Shell] Fixed exception type.
[Shell] Merging arguments for cmd launch to let single command components be passed as different arguments.
Мусин Камиль, 295 группа, Калькулятор
shell error correction 3
Calculator
Calculator. Version 2.
Shell. Iakovenko Pavel, gr.297