Make package.json definitive source of version information

I noticed that the tagged version's banner still says v0.1.54 at the top: https://github.com/addyosmani/backbone.paginator/blob/v0.5/dist/backbone.paginator.min.js

This is because the version was defined twice (in `package.json` and in the actual code). This PR fixes that.

I've called the version 0.5.0 because `npm` needs the 3 digits.

Maybe we should bump right away to something like `0.5.1-dev`.