Support HTML in tabs title

[pkozlowski-opensource]

Based on PR #67 (see the discussion in this PR for more details).

[max-mykhailenko]

I need it. Waiting for new versions

[ajoslin]

Tried to implement it, got annoying test failures. Will try to finish it tomorrow...

[petebacondarwin]

Hi @ajoslin - I was thinking of implementing it along these lines: http://plnkr.co/edit/qdVrVi6GCofRXjVI8LGw?p=preview

[ajoslin]

Fancy! I just had a PaneController which the paneHeading directive called a function on to set some html.