Add mocha unit-tests for insert(), remove().

data_structures/set/(index, test/test).js
	-Add `"use strict"`s to both modules.
	-Add mocha unit-tests for `Set.insert()` and `Set.remove()`.
	-Fix newly identified bugs in the functions.
Add unit-tests for isMember(), length().

data_structures/set/test/test.js
	-Define unit-tests for `Set.isMember()` and `Set.length()`.