feat(tooltip): tooltip now accepts and sanitizes HTML content

The tooltip uses the `$sanitize` service on the content passed to the `tooltip` attribute and then binds that content in the template using `ng-bind-html-unsafe`. The reason I used the service instead of the `ng-bind-html` attribute was to be able to design the tests to run exclusively off the scope, rather than requiring DOM knowledge, which would simply break with any template customization by the user.

The ngSanitize module is now included during testing and a note was added to the requirements section of the docs to specify that module is required, but we may need to make it more clear.

Closes #142.