Remove package from packageMap when there are no versions left

This addresses issue #21. When the user issues a DELETE command for a package, the package is not removed from packageMap. This causes a problem later on when someone goes to the index page, and the package that was just removed causes this error:

```
{
code: "InternalError",
message: "Cannot read property 'data' of undefined"
}
```

I fixed the problem by removing the package from packageMap entirely IF there are no versions installed. Accessing the index page works fine.