Begin writing unit tests: add main entry point.

test/test.js
	-Add the main entry point for all unit-tests.

package.json
	-Add test dependencies.
Add unit-test module for /lib/mapper.js.

test/mapper.js
	-Add a unit-test module for the `/lib/mapper` module.
	-Define a test for `mapper.getPropVal()`.
	-Add documentation comment to `getPropVal` and module header.
Define a test for createMapper() String fields.

test/mapper.js
	-Define `createMapperFields()`, which tests whether
	`createMapper()` properly remaps String fields.
Define test for createMapper() keep().

test/mapper.js
	-Define `createMapperKeep()`, which tests whether
	`createMapper()` discards input objects according to its mapper
	configuration's `keep()` function.
Heavily simplify test/mapper.js by abstraction.

test/mapper.js
	-There existed a significant amount of redundant (almost
	identical) code in the `createMapperFields()` and
	`createMapperKeep()` tests, used to create a mapper stream,
	write to it, and test its output.
	-Abstract it away into a function `testCreateMapper()`.
Define tests for coalesce/constant object mapping.

lib/mapper.js
	-Define functions to test whether the Mapper stream correctly
	maps fields according to `coalesce` and `constant` objects.
Strip a repeated clause from doc comments.

test/mapper.js
	-Most function documentation comments repeated an explanation of
	how they tested the Mapper stream functionality.
	-Superfluous/redundant, so remove.
Define createMapperFunction().

test/mapper.js
	-Test whether function fields are correctly interpreted by the
	mapper.
Merge branch 'feature/tests' into develop

test/(mapper, test).js, package.json
	-Add TAP unit-tests, primarily for the `lib/mapper` module.