Gitignore fixtures and dist/commonjs.

Fixes #1.

Gitignores:

- fixtures: pretty sure it should be ignored
- dist/commonjs. Please confirm. I imagine it should because only `dist/octokat.js` needs to be Git tracked for bower, while the CommonJS targets NPM Nodejs users.

This requires the addition of a `.npmignore` so that NPM won't use the `.gitignore` which would ignore `dist/commonjs/`. `package.json` is never ignored, so `*` works.

Tested locally with `npm  install -g .` and worked.