App icon - close #87

This PR has 2 consequences:
* when a user adds bestnest as a home screen bookmark on ios they will get a nice app icon for their home screen
* when a user opens the resulting bookmark, the safari user interface components will be hidden to make it appear even more like a native application

The changes required for this are pretty simple:
1. new grunt task `img-dev` to make sure images are available after the build
2. `<meta>` tags added to the root `index.html` to tell ios where to find the images
should be added to the `build-release` task, too