Add a second subset-generation algorithm.

algorithms/generate_subsets_2/README.md
	-Add an brief explanatory README.

algorithms/generate_subsets_2/(index, test/test).js
	-Add a complete implementation of a second algorithm that
	finds the power set of a set.
Add more to README, rename module to power_set_2.

algorithms/generate_subsets_2/README.md
	-Describe the contents of the module, and show example usage.

algorithms/generate_subsets_2/(index, test/test).js
	-Change references to `generate_subsets_2` and
	`generateSubsets()` to `power_set_2` and `powerSet()`.
Move generate_subsets_2/ -> power_set_2/.

algorithms/(generate_subsets_2 -> power_set_2)/
	-Rename the `generate_subsets_2/` directory to `power_set_2/`,
	to reflect the module name-change.
Merge branch 'algorithms/subsets_2' into develop

algorithms/power_set_2/
	-Implement a second power-set algorithm.