0.3.4

Lots of small changes.

### Breaking API changes
- `value` is no longer an instance property, replaced by `_value` and `setValue` and `getValue`, the latter two being the preferred means of setting and accessing the property.

### Potentially breaking API changes
- `_value` is initially set to `0` rather than `undefined`

### Enhancements
- `mutator` is available as a instance method, in case you want to reuse it
- The logic for type detection of the arguments is clearer and has explanatory comments
- `package.json` is now checked in and has a `prepublish` hook so this will work from NPM out of the box
- Dumb `build.sh` now replaced by a `Makefile`.