feat(tooltip): added tooltip-html-unsafe directive

The directive displays the unsanitized HTML in the tooltip instead of
the escaped text.

The $tooltip service has been modified to allow a little more
flexibility in terms of the prefix used on the $observe'd attributes.
For example, the `tooltip-html-unsafe` directive needs to be called as
written, but it would be nonsensical to require all other attributes
(like animation or placement) to also use that verbose prefix as opposed
to the simpler and more familiar `tooltip-` prefix. The service now
allows independent specification of the name and its prefix.

Lastly, the docs for the tooltip and popover have been updated to show
their available optional attributes.

Closes #246