Fixed tooltip directive for BS 2 to 3 change.

Since Bootstrap v3.0 the tooltip stores its data in a 'bs.' prefixed data attribute.

Without this change, Chrome console reports issue:

    TypeError: Cannot set property 'show' of undefined
        at postLink ([...]/angular-strap.js:800:22)

See line differences:
https://github.com/twbs/bootstrap/blob/v3.0.0/js/tooltip.js#L367
https://github.com/twbs/bootstrap/blob/v2.3.2/js/bootstrap-tooltip.js#L331