schemeNumber.js (and bigNumber.js if necessary) should come in via npm.

Browserify should be capable of resolving the dependency if we `require()` it, then the reference can be to the distribution file alone.

There's probably also a 'bug' in having `xpath.html` refer to `src/models` as well as the distribution file, as the distribution file likely already includes the models (and if it doesn't, we should resolve that issue instead).