IE 8 Compatibility

Hi all,

I'm trying to address issues #8 and #93. I'm creating this PR just to have a place to discuss about what to do and how, the work is not done yet (obviously...)

Just for a start, I edited the demo page to see how it feels in IE 8

* inserted a polyfill https://code.google.com/p/ie7-js/ (found it here https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills), it's still in beta but it's one of those that contains everything, seemed a good choice; other suggestions?
* copied and edited "by hand" the `ieshiv.js` from angular-ui and inserted in the demo page as well

Many widgets seemed to start working correctly just with this little effort (see yourself)

**TODO** (just as they come to mind)
* For now I put the `ieshiv.js` between the demo-assets, as a next step I'd like to edit the `build` task and make it generate the shiv automatically. I think it should be released together with the framework, because it should stay in sync with the framework version, but it should be an external optional file (I mean, not minified with the framework) and so it should be moved from the demo-assets directory. Any idea where to put it? I don't want to put it inside the `src`, it's not a module, it's more of optional stuff.

* After that I was thinking about writing a little description in the demo page about how to include all the stuff needed to make ui-bootstrap run in IE8

* Last but not least, edit the widgets to turn the tests green in IE 8 too and verify how they behave graphically (I suppose this must be done by hand, literally examining every widget inside the browser, clicking here and there)

* Other things?