Add support for NPM releases

Adds the `grunt-release` npm package to `dev-dependencies` and updates the options to only push to npm (instead of bumping version, creating tags, pushing tags)

This fix is for: https://github.com/angular-ui/angular-google-maps/issues/964

Test: 
```bash
# Does a dry run
grunt release --no-write
``` 

Verify that the dry run only pushes to npm without anything extra.
I think that makes sense. I removed the publish task from bump so now you would have to use one of the bump tasks and then run `grunt release` separately.