Add README/filler index.js to power_set_3.

algorithms/power_set_3/(README.md, index.js)
	-Add a basic README and index.js to the third power-set
	implementation.
Add a completed implementation of power_set_3.

algorithms/power_set_3/index.js
	-Define/document a completed `powerSet()`.
Add unit-tests to the power_set_3 module.

algorithms/power_set_3/test/test.js
	-Add a (slightly flawed) unit-test for `powerSet()`.
Update README with sample usage/test documentation.

algorithms/power_set_3/README.md
	-Add example usage and a mention of the module's unit-test.
Merge branch 'algorithms/power_set_3' into develop

algorithms/power_set_3/
	-Implement a third power-set algorithm.
Fix bug in the power_set module.

algorithms/power_set/index.js
	-Incorrectly returned an empty set as the power-set of an empty
	set, while, in reality, it should've contained one empty set.