Added second task
Sandrikova Maria, 296, Calculator

Initial commit
Added Shell back on.
Added shell-specific exception class.
Added synchronizing with double checker for ShellReceiver singleton.
[Shell] Modified IO restrictions.
[Shell] Replaced Vector usages with ArrayList.
[Shell] Refactored 'cd' command implementation.
shell ver.1
[Shell] Fixed exception type.
[Shell] Merging arguments for cmd launch to let single command components be passed as different arguments.
Shell ver.2
Мусин Камиль, 295 группа, Калькулятор
Shell ver.2 (fixed little bugs)
Shell. Iakovenko Pavel, gr.297
Shell ver.3 (formatting code)
Shell ver.3 (cp <dir\file> <dir> fixed)
Shell ver.3 (enter bug fixed)
Shell ver.4 (fixed bug with spaces)
Minor code convention changes