1. Comments have be added to main.cpp
2. Fixed list::Delete() bug, which caused seg fault.
3. Input reading changed from line-by-line to space-separated.
4. Helper function for checking whether a token is indeed an integer.
1. Changed the signature of Delete from Delete(position_t):bool to
Delete(position_t):position_t
2. Altered the Delete implementation logic to return the next valid
object on the right after deletion of the current item pointed by the
input argument.