Added remove method

Remove method checks first if there are 0 element, only 1 element, or
only 2 elements in the list, and then do a while loop to find if data
== current->data.

PS: I hope the code does not confuse you as it is very long
Added Retrieve method

Edited Remove and Retrieve prototypes
Edited remove method

Added line of code (size--) every time data == data inside the list,
after removing, size should decrease by one.