Improvements for npm publishing. Fixes #232.

- Published package no longer includes lots of dev-only stuff.
- Published package now *does* include pkg/sinon.js and pkg/sinon-ie.js, since no longer is .gitignore being used as .npmignore
- Add a prepublish script to run the build script, to ensure that pkg/sinon.js and pkg/sinon-ie.js are present and up-to-date for publishing.