feat(tabs): Allow html in tab heading

I tried to do the pane heading way Pete did in accordion, but it really refused to work.  Check out [tabs_html_1 branch -> tabs.js](https://github.com/angular-ui/bootstrap/blob/cf6fa9a09a3eaa3f9568c6d25f04f66d46c4150f/src/tabs/tabs.js).  I thought the way with the `$timeout` was just bad, so ...

Instead, I just did transclusion on the `<pane>` directive and look for child `<pane-heading>` elements in there. Check out tabs.js on the PR.  

Thoughts?  Is this way good enough or too "hacky"?  Can someone play with the other branch and see if they could get it working?  I've messed with the other way for days and it just doesn't like me, haha.  I explained a lot in comments why the approach Pete did for accordion just wasn't working here: [tabs_html_2 branch -> tabs.js:L90](https://github.com/angular-ui/bootstrap/blob/cf6fa9a09a3eaa3f9568c6d25f04f66d46c4150f/src/tabs/tabs.js#L90)

Overall, I don't like either way that much (the one in this PR or in tabs_html_1): they are both kind of hacky.  But I do think this approach (tabs_html_2) is a bit better than tabs_html_1.