Allow null model value and ensure tinymce is instantiated

First fix just ensures that `undefined` doesn't get passed to `setContent` because I found that `$viewValue` can be null if using ngRequired directive.

Second fix uses an interval loop to keep checking for the tinymce editor object because otherwise `tinyInstance` can be undefined since `tinymce.init` is in a `setTimeout`.
Using tinymce.onInit to ensure content gets set after tinymce is set up
Merge pull request #8 from jrust/patch-1

Allow null model value and ensure tinymce is instantiated