Adding leaflet directive

Leaflet directive from https://github.com/eknuth/ionic-angular-cordova-leaflet-seed is now in www/js/directives.
To use it in HTML, simply make a `<leaflet>` tag, and make sure the out app module declaration looks something like `var app = angular.module('map-app', ['leaflet-directive'])`.

When we implement this, we will need to refactor how we initialize the map. It's being formatted on the page correctly, thanks to our CSS map id, but the current configuration doesn't actually add the buttons or initialize coordinates on San Francisco.

One small extra change, I added build files with `.xcuserstate` to our `.gitignore`, so that we don't commit this every time we rebuild the app for testing.