Simplify build process and published packages

1. Remove generated files from the repository.
This will eliminate the need for synchronizing `src/` and `lib/` directories by hand and lead to the cleaner git history.

2. Register "prepublish" and "test" npm scripts.
These scripts are standard interfaces NPM provides. "Prepublish" will fire any `npm publish` or `npm install`. Of course `xyz` is still managing the publishing, but it would be still better if NPM kept `lib/` in sync automatically for you.

3. Discard CoffeeScript sources and build files from published tarballs.
Leave CoffeeScript for development and publish the compiled JS only. Users don't need any CoffeeScript.
The package itself becomes tighter:

```bash
$ npm pack
 
> string-format@0.2.1 prepublish .
> make

node_modules/.bin/coffee --compile --output lib -- src/string-format.coffee
string-format-0.2.1.tgz

$ tar tf string-format-0.2.1.tgz
package/package.json
package/.npmignore
package/README.md
package/LICENSE
package/lib/string-format.js
```

What do you think?
One last change: let's `"make clean && make"` so we recreate the JavaScript file even if for some reason its modification time is later than that of the source file.