Add operation module, def Operation/BinaryOp.

books/nand2tetris/projects/7_virtual_machine_1_stack_arithmetic/
vmtranslator/operation.py
	-Add the `operation` module, which will contain classes and
	functions for dealing with all VM arithmetic/logic operations.
	-Define the `Operation` abstract base class, and `BinaryOp`;
	thoroughly document each.
	-Add docstring to module header.