Add leaflet modules to bower.json; add resolution for D3; grunt run script

This PR adds the leaflet and leaflet.markercluster modules to bower.json, along with a version resolution for `d3`. I was getting Grunt errors without these changes.

Also, I added a "grunt" command to `scripts` so you can `npm run grunt` instead of having to having to `npm install -g grunt` to run `grunt`. Let me know if you want me to split or remove that commit.