Feature(#348): dynamic item type

#348 requests to be able to create dividers dynamically. Using this pull, item-type can be defined in a similar way as ng-class using an angular expression, thus making it possible to change the type dynamically. For example, using the existing Ionic list example, with line 58 as
item ng-repeat="movie in movies" href="#/movie/{{movie.id}}" item-type="{'item-divider': (movie.rating > 7)}"

will result in the list seen in screenshot (all movies that have rating higher than 7 are bold)
![screenshot](https://f.cloud.github.com/assets/487758/1837886/2f5f7690-7448-11e3-929f-c47ae80251ec.png)

Note however that this is a breaking change as item-type="item-divider" for example won't work (must be item-type="'item-divider'" again following the same pattern as ng-class).