Add skeleton Set implementation, README.

data_structures/set/README.md
	-Add a short README, to contain much more content in the future.

data_structures/set/index.js
	-Add a skeleton implementation of a Set, structured as a Node
	package.
	-Define `Set()` with some member variables and methods declared,
	and a placeholder `test()`.
Def/doc insert(), remove(), length(), isMember().

data_structures/set/index.js
	-Define and document `insert()`, `remove()`, `length()`, and
	`isMember()`.
	-Add a documentation comment to the file header and `Set()`.
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()`.
Fix jshint errors in test/test.js

data_structures/set/test/test.js
	-Use `===` instead of `==` where possible.
Add varargs to Set(), insert(), remove().

data_structures/set/index.js
	-Redefine `Set()`, `Set.insert()`, and `Set.remove()` to accept
	any number of arguments.
	-Update documentation comments accordingly.

data_structures/set/test/test.js
	-Add new tests for the above.
Def union(), isEqual(); add doc/unit tests.

data_structures/set/(index, test/test).js
	-Define `Set.union()` and `Set.isEqual()`.
	-Add documentation comments and appropriate unit-tests.
Def, doc, and test intersection() and isSubset().

data_structures/set/(index, test/test).js
	-Define `Set.intersection()` and `Set.isSubset()`.
	-Add documentation comments and unit-tests.
Def, doc, and test difference().

data_structures/set/(index, test/test).js
	-Define and document `Set.difference()`.
	-Add a unit-test.
Add content to README; def and test toString().

data_structures/set/README.md
	-Add a README containing a summary of the contents of this
	project, the functionality provided by the `Set` module, and a
	usage example.

data_structures/set/(index, test/test).js
	-Define and document `toString()`.
	-Add a unit-test.
Merge branch 'data_structures/set' into develop

data_structures/set/
	-Implement the set data-structure.