Replace build.sh with simple Makefile
Add package.json file with prepublish entry (closes #13)
Bump version to 0.3.4
Add isFunc and isObject convenience methods.
Move logic for creating the random value generator(s) out of the class

Instead, move them into private methods. All instance methods should
be user-facing.
Clean up constructor API logic and add explanatory comments to it
Underscore-prefix value property and add accessor/setter methods
Update README to reflect `value` changes

Because using accessor/setter is preferred, and because `value` is 0
by default.
Save mutator function on the instance so it can be reused if wanted