feat($ionicToast): service for displaying toast messages

I noticed a few people have asked about Toast messages that are predominantly used in native Android apps. My project needed something similar so i thought i would out.

![toast-demo](https://cloud.githubusercontent.com/assets/760516/2557265/08c6f66c-b6fa-11e3-9929-3cf3421b582b.gif)

If you decide to merge, copy over the the gif to your amazon s3 server and update the docs in `service/ionicToast.js` which references the imgur link !(http://i.imgur.com/xF4pRpT.gif)