Added WordPress shortcode wrapper for existing widget.

Very simple addition to add a shortcode to the existing widget.

Invoking the shortcode on a page looks like:
```
[ravelry-projects username='my-user-id' finished_projects=1 ravelry_profile_link=0 nr_projects=20 project_notes=0]
```

There are defaults in place: 5 projects including unfinished items, include the project notes and a link to your profile.  The minimalist form of the shortcode only needs a username to be useful:
```
[ravelry-projects username='my-user-id']
```