Add Starter Form

This pull request does what was discussed in #98.

On a technical level it checks if a form exists when Ninja Forms is activated. If there are no existing forms then it loads a simple contact form.

I was also able to refactor some code in the `includes/admin/pages/ninja-forms-impexp/tabs/impexp-forms/impexp-forms.php` file and put it into more general file, `includes/import-export.php`.