Move set_cover.generateSubsets() to its own module.

algorithms/(set_cover, generate_subsets)/index.js
	-Move `generateSubsets()` from `set_cover/index.js` to its own
	module, `generate_subsets/index.js`.
	-`require()` in the module for use in `findSetCover()`.
Add unit-tests to generate_subsets.

algorithms/generate_subsets/(index, test/test).js
	-Remove `testSetCover()` from `index.js`.
	-Add Mocha unit-tests to the `generate_subsets` subset module.
Add a README to generate_subsets.

algorithms/generate_subsets/README.md
	-Add a README describing the `generate_subsets` module's
	contents and showing sample usage.
Add tests to set_cover module. Rem testSetCover().

algorithms/set_cover/index.js
	-Remove `testSetCover()`, which served mostly as a
	human-oriented example, in favor of machine-oriented
	unit-tests.

algorithms/set_cover/test/test.js
	-Add Mocha unit-tests.
Add content to set_cover README.

algorithms/set_cover/README.md
	-Add a description of the module's contents, and document
	sample usage.
Merge branch 'algorithms/subsets' into develop

algorithms/(generate_subsets, set_cover)/
	-Move the subset-generation algorithm from `set_cover` to its
	own module, `generate_subsets`.
	-Add unit-tests to both, and descriptive READMEs.