Generate Assetic packages only when content for scripts and/or styles isn't empty

This PR fixes #58. 

### Description
When the `bower.json` of a component doesn't specify a `main`-directive, the command (and therefor the bundle) cannot find the correct files for creating the Assetic packages to be used in the template (e.g. `@jquery_js`). Using these definitions results in empty assets delivered by Assetic.

### Solution
This PR solves this problem by checking if the packages to created have content. If not, the creation is stopped for a particular package.
The command `sp:bower:list` now only print out packages with found files. All other assets need to be included manually.

**Be social:** When you include a package without a correct defined `main`-directive, please be social and create a PR in the respective project to include it. :+1:
This way the bundle and bower will be even easier to use.

### Todo
The following things still need to do:
- [ ] Adjust documentation
- [ ] Fix tests
- [ ] Add BC note (?)